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/apis/common/responses.py

@@ -1,36 +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 enum import Enum
-from typing import Any
-
-from pydantic import BaseModel
-
-from llama_stack.schema_utils import json_schema_type
-
-
-class Order(Enum):
-    """Sort order for paginated responses.
-    :cvar asc: Ascending order
-    :cvar desc: Descending order
-    """
-
-    asc = "asc"
-    desc = "desc"
-
-
-@json_schema_type
-class PaginatedResponse(BaseModel):
-    """A generic paginated response that follows a simple format.
-
-    :param data: The list of items for the current page
-    :param has_more: Whether there are more items available after this set
-    :param url: The URL for accessing this list
-    """
-
-    data: list[dict[str, Any]]
-    has_more: bool
-    url: str | None = None