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/strong_typing/mapping.py

@@ -0,0 +1,39 @@
+# 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.
+
+"""
+Type-safe data interchange for Python data classes.
+
+:see: https://github.com/hunyadi/strong_typing
+"""
+
+import keyword
+
+from .auxiliary import Alias
+from .inspection import get_annotation
+
+
+def python_field_to_json_property(python_id: str, python_type: object | None = None) -> str:
+    """
+    Map a Python field identifier to a JSON property name.
+
+    Authors may use an underscore appended at the end of a Python identifier as per PEP 8 if it clashes with a Python
+    keyword: e.g. `in` would become `in_` and `from` would become `from_`. Remove these suffixes when exporting to JSON.
+
+    Authors may supply an explicit alias with the type annotation `Alias`, e.g. `Annotated[MyType, Alias("alias")]`.
+    """
+
+    if python_type is not None:
+        alias = get_annotation(python_type, Alias)
+        if alias:
+            return alias.name
+
+    if python_id.endswith("_"):
+        id = python_id[:-1]
+        if keyword.iskeyword(id):
+            return id
+
+    return python_id