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/exception.py

@@ -0,0 +1,23 @@
+# 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
+"""
+
+
+class JsonKeyError(Exception):
+    "Raised when deserialization for a class or union type has failed because a matching member was not found."
+
+
+class JsonValueError(Exception):
+    "Raised when (de)serialization of data has failed due to invalid value."
+
+
+class JsonTypeError(Exception):
+    "Raised when deserialization of data has failed due to a type mismatch."