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 .`

llama_stack/providers/remote/inference/passthrough/config.py

@@ -1,34 +0,0 @@
-# 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 Any
-
-from pydantic import Field, SecretStr
-
-from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
-from llama_stack.schema_utils import json_schema_type
-
-
-@json_schema_type
-class PassthroughImplConfig(RemoteInferenceProviderConfig):
-    url: str = Field(
-        default=None,
-        description="The URL for the passthrough endpoint",
-    )
-
-    api_key: SecretStr | None = Field(
-        default=None,
-        description="API Key for the passthrouth endpoint",
-    )
-
-    @classmethod
-    def sample_run_config(
-        cls, url: str = "${env.PASSTHROUGH_URL}", api_key: str = "${env.PASSTHROUGH_API_KEY}", **kwargs
-    ) -> dict[str, Any]:
-        return {
-            "url": url,
-            "api_key": api_key,
-        }