refactor: enforce top-level imports for llama-stack-api

Enforce that all imports from llama-stack-api use the form:

from llama_stack_api import <symbol>

 This prevents external code from accessing internal package structure
 (e.g., llama_stack_api.agents, llama_stack_api.common.*) and establishes
 a clear public API boundary.

 Changes:
 - Export 400+ symbols from llama_stack_api/__init__.py
 - Include all API types, common utilities, and strong_typing helpers
 - Update files across src/llama_stack, docs/, tests/, scripts/
 - Convert all submodule imports to top-level imports
 - ensure docs use the proper importing structure

 Addresses PR review feedback requiring explicit __all__ definition to
 prevent "peeking inside" the API package.

Signed-off-by: Charlie Doern <cdoern@redhat.com>

.github/workflows/python-build-test.yml

@@ -30,14 +30,14 @@ jobs:
         activate-environment: true
         version: 0.7.6
 
-    - name: Build Llama Stack Spec package
+    - name: Build Llama Stack API package
       working-directory: src/llama-stack-api
       run: uv build
 
     - name: Build Llama Stack package
       run: uv build
 
-    - name: Install Llama Stack package (with spec from local build)
+    - name: Install Llama Stack package (with api stubs from local build)
       run: |
         uv pip install --find-links src/llama-stack-api/dist dist/*.whl