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/models.py

@@ -9,6 +9,8 @@ import threading
 from dataclasses import dataclass, field
 from typing import Dict, Any, Optional
 
+from llm_connect.exceptions import LLMBudgetExceededError
+
 
 class BudgetTracker:
     """Shared token budget for a call or delegation chain.
@@ -36,8 +38,6 @@ class BudgetTracker:
 
     def consume(self, tokens: int) -> None:
         """Record *tokens* as spent. Raises ``LLMBudgetExceededError`` if cap exceeded."""
-        from llm_connect.exceptions import LLMBudgetExceededError  # avoid circular at module load
-
         with self._lock:
             new_spent = self.spent + tokens
             if new_spent > self.total:
@@ -46,7 +46,6 @@ class BudgetTracker:
                     total=self.total,
                     spent=self.spent,
                     requested=tokens,
-                    context={"total": self.total, "spent": self.spent, "requested": tokens},
                 )
             self.spent = new_spent