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/gemini.py

@@ -2,7 +2,6 @@
 Google Gemini adapter — calls the Generative Language REST API directly.
 """
 
-import asyncio
 import time
 from typing import Optional, Dict, Any
 
@@ -111,10 +110,6 @@ class GeminiAdapter(LLMAdapter):
         self._consume_budget(config, response)
         return response
 
-    async def async_execute_prompt(self, prompt: str, config: RunConfig) -> LLMResponse:
-        """Async wrapper — runs execute_prompt in a thread executor."""
-        return await asyncio.to_thread(self.execute_prompt, prompt, config)
-
     def validate_config(self, config: RunConfig) -> bool:
         if not self._api_key:
             return False