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/providers/remote/inference/runpod/config.py

@@ -0,0 +1,32 @@
+# 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 RunpodImplConfig(RemoteInferenceProviderConfig):
+    url: str | None = Field(
+        default=None,
+        description="The URL for the Runpod model serving endpoint",
+    )
+    auth_credential: SecretStr | None = Field(
+        default=None,
+        alias="api_token",
+        description="The API token",
+    )
+
+    @classmethod
+    def sample_run_config(cls, **kwargs: Any) -> dict[str, Any]:
+        return {
+            "url": "${env.RUNPOD_URL:=}",
+            "api_token": "${env.RUNPOD_API_TOKEN}",
+        }