refactor(src): eliminate 11 T | None legacy wrappers in favor of _result API

TIER-3 READ AGENTS.md + conductor/workflow.md + conductor/code_styleguides/error_handling.md + the 4 source files + 3 test files before this commit.

The code_path_audit_phase_2_20260624 track (Tier 2) shipped 11 audit
fixes (4 NG1 + 7 NG2) but used a heuristic bypass for 4 of the NG2
wrappers: legacy T | None functions that exist only to maintain test
patcher compatibility. Per the review at
docs/reports/REVIEW_TIER2_code_path_audit_phase_2_20260624.md Finding 8,
this track eliminates the legacy wrappers properly.

11 wrappers eliminated (8 main + 3 _legacy_compat inner):
- src/ai_client.py: get_current_tier (1 src + 1 test consumer)
- src/ai_client.py: _gemini_tool_declaration + _legacy_compat (2 test consumers)
- src/ai_client.py: run_tier4_patch_callback + _legacy_compat (was 0 direct callers
  but had 2 callback references in app_controller/multi_agent_conductor;
  callback contract migrated to Callable[[str, str], Result[str]] instead of
  preserving an Optional[str] adapter)
- src/mcp_client.py: _get_symbol_node + _legacy_compat (8 in-file consumers)
- src/mcp_client.py: find_in_scope (nested inside _get_symbol_node_result;
  private impl detail, audit doesn't catch T | None, left as-is)
- src/external_editor.py: launch_diff (1 src + 3 test + 1 live_gui test consumer)
- src/external_editor.py: launch_editor (no consumers; deleted)
- src/session_logger.py: log_tool_output (2 src + 3 test consumers)
- src/project_manager.py: parse_ts (no consumers; deleted)

For each consumer: replace legacy_fn(args) with legacy_fn_result(args).data.
For T | None checks: replace if x is None: with if not result.ok: or
if not result.ok or not isinstance(result.data, ...) (depending on pattern).

For run_tier4_patch_callback specifically: the wrapper was a callback adapter
(not a backward-compat shim) and had 2 callback references as consumers.
Rather than keep the adapter (which would re-introduce the Optional[str]
return that the strict audit catches), the patch_callback contract was migrated
from Callable[[str, str], Optional[str]] to Callable[[str, str], Result[str]]
in shell_runner.py + app_controller.py + 9 _send_<vendor>_result signatures
in ai_client.py. This propagates the Result[str] through the callback and
lets shell_runner unwrap with if r.ok and r.data instead of if patch_text.

Verification:
- audit_optional_in_3_files --strict: 0 return-type Optional[T] (down from 1)
- audit_exception_handling --strict: 0 violations (unchanged)
- audit_legacy_wrappers: 0 legacy wrappers (unchanged)
- 15 affected test files: 168 tests pass
- 8 mcp_client/structural/baseline test files: 55 tests pass
- 3 session/gui test files: 7 tests pass
- 0 return-type Optional[T] in src/ai_client.py (was 1: run_tier4_patch_callback)

tests/test_session_logger_optimization.py

@@ -78,20 +78,23 @@ def test_log_tool_output_saves_in_session_outputs(temp_session_setup: tuple[Path
  output_content = "This is some tool output content."
  
  # Call log_tool_output
- output_path_str = session_logger.log_tool_output(output_content)
- assert output_path_str is not None
- 
- output_path = Path(output_path_str)
+ output_result = session_logger.log_tool_output_result(output_content)
+ assert output_result.ok, f"log_tool_output failed: {output_result.errors}"
+ assert output_result.data is not None
+
+ output_path = Path(output_result.data)
  assert output_path.parent == outputs_subdir
  assert output_path.name == "output_0001.txt"
  assert output_path.read_text(encoding="utf-8") == output_content
- 
+
  # Verify second call increments sequence
- output_path_str_2 = session_logger.log_tool_output("More content")
- assert output_path_str_2 is not None
- assert Path(output_path_str_2).name == "output_0002.txt"
+ output_result_2 = session_logger.log_tool_output_result("More content")
+ assert output_result_2.ok, f"log_tool_output failed: {output_result_2.errors}"
+ assert output_result_2.data is not None
+ assert Path(output_result_2.data).name == "output_0002.txt"
 
 def test_log_tool_output_returns_none_if_no_session(temp_session_setup: tuple[Path, Path]) -> None:
  # We don't call open_session here
- output_path_str = session_logger.log_tool_output("Should not save")
- assert output_path_str is None
+ output_result = session_logger.log_tool_output_result("Should not save")
+ assert not output_result.ok
+ assert output_result.data is None