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/apis/inspect/inspect.py

@@ -0,0 +1,94 @@
+# 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 typing import Protocol, runtime_checkable
+
+from pydantic import BaseModel
+
+from llama_stack.apis.version import LLAMA_STACK_API_V1
+from llama_stack.providers.datatypes import HealthStatus
+from llama_stack.schema_utils import json_schema_type, webmethod
+
+
+@json_schema_type
+class RouteInfo(BaseModel):
+    """Information about an API route including its path, method, and implementing providers.
+
+    :param route: The API endpoint path
+    :param method: HTTP method for the route
+    :param provider_types: List of provider types that implement this route
+    """
+
+    route: str
+    method: str
+    provider_types: list[str]
+
+
+@json_schema_type
+class HealthInfo(BaseModel):
+    """Health status information for the service.
+
+    :param status: Current health status of the service
+    """
+
+    status: HealthStatus
+
+
+@json_schema_type
+class VersionInfo(BaseModel):
+    """Version information for the service.
+
+    :param version: Version number of the service
+    """
+
+    version: str
+
+
+class ListRoutesResponse(BaseModel):
+    """Response containing a list of all available API routes.
+
+    :param data: List of available route information objects
+    """
+
+    data: list[RouteInfo]
+
+
+@runtime_checkable
+class Inspect(Protocol):
+    """Inspect
+
+    APIs for inspecting the Llama Stack service, including health status, available API routes with methods and implementing providers.
+    """
+
+    @webmethod(route="/inspect/routes", method="GET", level=LLAMA_STACK_API_V1)
+    async def list_routes(self) -> ListRoutesResponse:
+        """List routes.
+
+        List all available API routes with their methods and implementing providers.
+
+        :returns: Response containing information about all available routes.
+        """
+        ...
+
+    @webmethod(route="/health", method="GET", level=LLAMA_STACK_API_V1, require_authentication=False)
+    async def health(self) -> HealthInfo:
+        """Get health status.
+
+        Get the current health status of the service.
+
+        :returns: Health information indicating if the service is operational.
+        """
+        ...
+
+    @webmethod(route="/version", method="GET", level=LLAMA_STACK_API_V1, require_authentication=False)
+    async def version(self) -> VersionInfo:
+        """Get version.
+
+        Get the version of the service.
+
+        :returns: Version information containing the service version number.
+        """
+        ...