chore(package): migrate to src/ layout (#3920)

Migrates package structure to src/ layout following Python packaging
best practices.

All code moved from `llama_stack/` to `src/llama_stack/`. Public API
unchanged - imports remain `import llama_stack.*`.

Updated build configs, pre-commit hooks, scripts, and GitHub workflows
accordingly. All hooks pass, package builds cleanly.

**Developer note**: Reinstall after pulling: `pip install -e .`

src/llama_stack/core/client.py

@@ -0,0 +1,205 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import inspect
+import json
+import sys
+from collections.abc import AsyncIterator
+from enum import Enum
+from typing import Any, Union, get_args, get_origin
+
+import httpx
+from pydantic import BaseModel, parse_obj_as
+from termcolor import cprint
+
+from llama_stack.providers.datatypes import RemoteProviderConfig
+
+_CLIENT_CLASSES = {}
+
+
+async def get_client_impl(protocol, config: RemoteProviderConfig, _deps: Any):
+    client_class = create_api_client_class(protocol)
+    impl = client_class(config.url)
+    await impl.initialize()
+    return impl
+
+
+def create_api_client_class(protocol) -> type:
+    if protocol in _CLIENT_CLASSES:
+        return _CLIENT_CLASSES[protocol]
+
+    class APIClient:
+        def __init__(self, base_url: str):
+            print(f"({protocol.__name__}) Connecting to {base_url}")
+            self.base_url = base_url.rstrip("/")
+            self.routes = {}
+
+            # Store routes for this protocol
+            for name, method in inspect.getmembers(protocol):
+                if hasattr(method, "__webmethod__"):
+                    sig = inspect.signature(method)
+                    self.routes[name] = (method.__webmethod__, sig)
+
+        async def initialize(self):
+            pass
+
+        async def shutdown(self):
+            pass
+
+        async def __acall__(self, method_name: str, *args, **kwargs) -> Any:
+            assert method_name in self.routes, f"Unknown endpoint: {method_name}"
+
+            # TODO: make this more precise, same thing needs to happen in server.py
+            is_streaming = kwargs.get("stream", False)
+            if is_streaming:
+                return self._call_streaming(method_name, *args, **kwargs)
+            else:
+                return await self._call_non_streaming(method_name, *args, **kwargs)
+
+        async def _call_non_streaming(self, method_name: str, *args, **kwargs) -> Any:
+            _, sig = self.routes[method_name]
+
+            if sig.return_annotation is None:
+                return_type = None
+            else:
+                return_type = extract_non_async_iterator_type(sig.return_annotation)
+                assert return_type, f"Could not extract return type for {sig.return_annotation}"
+
+            async with httpx.AsyncClient() as client:
+                params = self.httpx_request_params(method_name, *args, **kwargs)
+                response = await client.request(**params)
+                response.raise_for_status()
+
+                j = response.json()
+                if j is None:
+                    return None
+                # print(f"({protocol.__name__}) Returning {j}, type {return_type}")
+                return parse_obj_as(return_type, j)
+
+        async def _call_streaming(self, method_name: str, *args, **kwargs) -> Any:
+            webmethod, sig = self.routes[method_name]
+
+            return_type = extract_async_iterator_type(sig.return_annotation)
+            assert return_type, f"Could not extract return type for {sig.return_annotation}"
+
+            async with httpx.AsyncClient() as client:
+                params = self.httpx_request_params(method_name, *args, **kwargs)
+                async with client.stream(**params) as response:
+                    response.raise_for_status()
+
+                    async for line in response.aiter_lines():
+                        if line.startswith("data:"):
+                            data = line[len("data: ") :]
+                            try:
+                                data = json.loads(data)
+                                if "error" in data:
+                                    cprint(data, color="red", file=sys.stderr)
+                                    continue
+
+                                yield parse_obj_as(return_type, data)
+                            except Exception as e:
+                                cprint(f"Error with parsing or validation: {e}", color="red", file=sys.stderr)
+                                cprint(data, color="red", file=sys.stderr)
+
+        def httpx_request_params(self, method_name: str, *args, **kwargs) -> dict:
+            webmethod, sig = self.routes[method_name]
+
+            parameters = list(sig.parameters.values())[1:]  # skip `self`
+            for i, param in enumerate(parameters):
+                if i >= len(args):
+                    break
+                kwargs[param.name] = args[i]
+
+            # Get all webmethods for this method (supports multiple decorators)
+            webmethods = getattr(method, "__webmethods__", [])
+
+            if not webmethods:
+                raise RuntimeError(f"Method {method} has no webmethod decorators")
+
+            # Choose the preferred webmethod (non-deprecated if available)
+            preferred_webmethod = None
+            for wm in webmethods:
+                if not getattr(wm, "deprecated", False):
+                    preferred_webmethod = wm
+                    break
+
+            # If no non-deprecated found, use the first one
+            if preferred_webmethod is None:
+                preferred_webmethod = webmethods[0]
+
+            url = f"{self.base_url}/{preferred_webmethod.level}/{preferred_webmethod.route.lstrip('/')}"
+
+            def convert(value):
+                if isinstance(value, list):
+                    return [convert(v) for v in value]
+                elif isinstance(value, dict):
+                    return {k: convert(v) for k, v in value.items()}
+                elif isinstance(value, BaseModel):
+                    return json.loads(value.model_dump_json())
+                elif isinstance(value, Enum):
+                    return value.value
+                else:
+                    return value
+
+            params = {}
+            data = {}
+            if webmethod.method == "GET":
+                params.update(kwargs)
+            else:
+                data.update(convert(kwargs))
+
+            ret = dict(
+                method=webmethod.method or "POST",
+                url=url,
+                headers={
+                    "Accept": "application/json",
+                    "Content-Type": "application/json",
+                },
+                timeout=30,
+            )
+            if params:
+                ret["params"] = params
+            if data:
+                ret["json"] = data
+
+            return ret
+
+    # Add protocol methods to the wrapper
+    for name, method in inspect.getmembers(protocol):
+        if hasattr(method, "__webmethod__"):
+
+            async def method_impl(self, *args, method_name=name, **kwargs):
+                return await self.__acall__(method_name, *args, **kwargs)
+
+            method_impl.__name__ = name
+            method_impl.__qualname__ = f"APIClient.{name}"
+            method_impl.__signature__ = inspect.signature(method)
+            setattr(APIClient, name, method_impl)
+
+    # Name the class after the protocol
+    APIClient.__name__ = f"{protocol.__name__}Client"
+    _CLIENT_CLASSES[protocol] = APIClient
+    return APIClient
+
+
+# not quite general these methods are
+def extract_non_async_iterator_type(type_hint):
+    if get_origin(type_hint) is Union:
+        args = get_args(type_hint)
+        for arg in args:
+            if not issubclass(get_origin(arg) or arg, AsyncIterator):
+                return arg
+    return type_hint
+
+
+def extract_async_iterator_type(type_hint):
+    if get_origin(type_hint) is Union:
+        args = get_args(type_hint)
+        for arg in args:
+            if issubclass(get_origin(arg) or arg, AsyncIterator):
+                inner_args = get_args(arg)
+                return inner_args[0]
+    return None