Add extensible decorator and cache controls

Introduce an extensible extension point system and finer cache control. Added extensible decorator in python/helpers/extension.py to wrap sync/async functions, emit start/end extension points, detect Agent instances, and allow extensions to modify inputs/results/exceptions. Reworked extension loading to use the cache API and a dedicated cache area. Enhanced python/helpers/cache.py with global and per-area toggles and early-return behavior in add/get/remove. Updated API handler caching (python/helpers/api.py) to use the cache area toggle and simplify cache logic. Marked several initialization functions as @extension.extensible in initialize.py. Moved nest_asyncio.apply() from agent.py into python/helpers/runtime.py to ensure the event loop patch is applied at runtime initialization. Also applied small typing/formatting tweaks and path handling improvements.

python/helpers/runtime.py

@@ -8,6 +8,9 @@ import asyncio
 import threading
 import queue
 import sys
+import nest_asyncio
+
+nest_asyncio.apply()
 
 T = TypeVar("T")
 R = TypeVar("R")