refactor: simplify post-WP-0002 cleanup

- Remove redundant async_execute_prompt overrides from OpenAI/Gemini/OpenRouter
  adapters (identical to base class default — asyncio import also removed)
- Cache prompt.split() result in MockLLMAdapter to avoid double evaluation
- Promote deferred LLMBudgetExceededError imports to module level in
  models.py and adapter.py (no circular dependency)
- Auto-populate context dict in LLMBudgetExceededError.__init__ so callers
  need not pass redundant context= kwarg

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

llm_connect/exceptions.py

@@ -82,6 +82,8 @@ class LLMBudgetExceededError(LLMError):
         cause: Optional[Exception] = None,
         context: Optional[Dict[str, Any]] = None,
     ):
+        if context is None:
+            context = {"total": total, "spent": spent, "requested": requested}
         super().__init__(message, cause=cause, context=context)
         self.total = total
         self.spent = spent