feat(tier4): Add patch generation for auto-patching

- Add TIER4_PATCH_PROMPT to mma_prompts.py with unified diff format
- Add run_tier4_patch_generation function to ai_client.py
- Import mma_prompts in ai_client.py
- Add unit tests for patch generation

tests/test_tier4_patch_generation.py

@@ -0,0 +1,67 @@
+from unittest.mock import MagicMock, patch
+import pytest
+from src import ai_client
+from src import mma_prompts
+
+def test_tier4_patch_prompt_exists() -> None:
+  """Test that TIER4_PATCH_PROMPT is defined in mma_prompts.py."""
+  assert hasattr(mma_prompts, "TIER4_PATCH_PROMPT")
+  prompt = mma_prompts.TIER4_PATCH_PROMPT
+  assert "unified diff" in prompt.lower() or "diff -u" in prompt.lower()
+  assert "---" in prompt
+  assert "+++" in prompt
+  assert "@@" in prompt
+
+def test_tier4_patch_prompt_format_instructions() -> None:
+  """Test that the patch prompt includes format instructions for unified diff."""
+  prompt = mma_prompts.TIER4_PATCH_PROMPT
+  assert "--- a/" in prompt or "---" in prompt
+  assert "+++ b/" in prompt or "+++" in prompt
+
+def test_run_tier4_patch_generation_exists() -> None:
+  """Test that run_tier4_patch_generation function exists in ai_client."""
+  assert hasattr(ai_client, "run_tier4_patch_generation")
+  assert callable(ai_client.run_tier4_patch_generation)
+
+def test_run_tier4_patch_generation_empty_error() -> None:
+  """Test that run_tier4_patch_generation returns empty string on empty error."""
+  with patch("src.ai_client._ensure_gemini_client"), \
+     patch("src.ai_client._gemini_client") as mock_client:
+    mock_resp = MagicMock()
+    mock_resp.text = ""
+    mock_client.models.generate_content.return_value = mock_resp
+    
+    result = ai_client.run_tier4_patch_generation("", "file context")
+    assert result == ""
+
+def test_run_tier4_patch_generation_calls_ai() -> None:
+  """Test that run_tier4_patch_generation calls the AI with the correct prompt."""
+  with patch("src.ai_client._ensure_gemini_client"), \
+     patch("src.ai_client._gemini_client", create=True) as mock_client, \
+     patch("src.ai_client.types") as mock_types:
+    mock_resp = MagicMock()
+    mock_resp.text = "--- a/test.py\n+++ b/test.py\n@@ -1 +1 @@\n-old\n+new"
+    mock_client.models.generate_content.return_value = mock_resp
+    mock_types.GenerateContentConfig = MagicMock()
+    
+    error = "TypeError: unsupported operand"
+    file_context = "def foo():\n    pass"
+    result = ai_client.run_tier4_patch_generation(error, file_context)
+    
+    mock_client.models.generate_content.assert_called()
+
+def test_run_tier4_patch_generation_returns_diff() -> None:
+  """Test that run_tier4_patch_generation returns diff text."""
+  with patch("src.ai_client._ensure_gemini_client"), \
+     patch("src.ai_client._gemini_client", create=True) as mock_client, \
+     patch("src.ai_client.types") as mock_types:
+    expected_diff = "--- a/src/test.py\n+++ b/src/test.py\n@@ -10,5 +10,6 @@\n def test_func():\n-    old_value = 1\n+    old_value = 1\n+    new_value = 2"
+    mock_resp = MagicMock()
+    mock_resp.text = expected_diff
+    mock_client.models.generate_content.return_value = mock_resp
+    mock_types.GenerateContentConfig = MagicMock()
+    
+    result = ai_client.run_tier4_patch_generation("error", "context")
+    assert "---" in result
+    assert "+++" in result
+    assert "@@" in result