test(ai_cache_tracking): migrate to send_result() (Phase 2.2)

The test calls ai_client.send() but does not check the return value -
it only verifies the side effect on gemini cache stats. Migrating to
send_result() and asserting result.ok is enough.

Changes:
- Rename ai_client.send(...) to ai_client.send_result(...)
- Add assert result.ok (the return value is unused)
- Add "from src.result_types import Result" import

2 tests pass.

tests/test_ai_cache_tracking.py

@@ -1,6 +1,7 @@
 import unittest
 from unittest.mock import patch, MagicMock
 from src import ai_client
+from src.result_types import Result
 import time
 
 def test_gemini_cache_tracking() -> None:
@@ -44,11 +45,12 @@ def test_gemini_cache_tracking() -> None:
    mock_client.caches.list.return_value = [MagicMock(size_bytes=5000)]
    
    # Act
-   ai_client.send(
+   result = ai_client.send_result(
     md_content="Some long context that triggers caching",
     user_message="Hello",
     file_items=file_items
    )
+   assert result.ok
    
    # Assert
    stats = ai_client.get_gemini_cache_stats()