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/inspect.py

@@ -0,0 +1,86 @@
+# 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.
+
+from importlib.metadata import version
+
+from pydantic import BaseModel
+
+from llama_stack.apis.inspect import (
+    HealthInfo,
+    Inspect,
+    ListRoutesResponse,
+    RouteInfo,
+    VersionInfo,
+)
+from llama_stack.core.datatypes import StackRunConfig
+from llama_stack.core.external import load_external_apis
+from llama_stack.core.server.routes import get_all_api_routes
+from llama_stack.providers.datatypes import HealthStatus
+
+
+class DistributionInspectConfig(BaseModel):
+    run_config: StackRunConfig
+
+
+async def get_provider_impl(config, deps):
+    impl = DistributionInspectImpl(config, deps)
+    await impl.initialize()
+    return impl
+
+
+class DistributionInspectImpl(Inspect):
+    def __init__(self, config: DistributionInspectConfig, deps):
+        self.config = config
+        self.deps = deps
+
+    async def initialize(self) -> None:
+        pass
+
+    async def list_routes(self) -> ListRoutesResponse:
+        run_config: StackRunConfig = self.config.run_config
+
+        ret = []
+        external_apis = load_external_apis(run_config)
+        all_endpoints = get_all_api_routes(external_apis)
+        for api, endpoints in all_endpoints.items():
+            # Always include provider and inspect APIs, filter others based on run config
+            if api.value in ["providers", "inspect"]:
+                ret.extend(
+                    [
+                        RouteInfo(
+                            route=e.path,
+                            method=next(iter([m for m in e.methods if m != "HEAD"])),
+                            provider_types=[],  # These APIs don't have "real" providers - they're internal to the stack
+                        )
+                        for e, _ in endpoints
+                        if e.methods is not None
+                    ]
+                )
+            else:
+                providers = run_config.providers.get(api.value, [])
+                if providers:  # Only process if there are providers for this API
+                    ret.extend(
+                        [
+                            RouteInfo(
+                                route=e.path,
+                                method=next(iter([m for m in e.methods if m != "HEAD"])),
+                                provider_types=[p.provider_type for p in providers],
+                            )
+                            for e, _ in endpoints
+                            if e.methods is not None
+                        ]
+                    )
+
+        return ListRoutesResponse(data=ret)
+
+    async def health(self) -> HealthInfo:
+        return HealthInfo(status=HealthStatus.OK)
+
+    async def version(self) -> VersionInfo:
+        return VersionInfo(version=version("llama-stack"))
+
+    async def shutdown(self) -> None:
+        pass