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_gui_2_result.py

@@ -1033,7 +1033,7 @@ def test_phase_5_l1393_open_patch_in_external_editor_result_success():
  L1393 _open_patch_in_external_editor_result returns Result.ok=True on success.
 
  The helper wraps the external editor launch try/except in
- App._open_patch_in_external_editor. On success (launcher.launch_diff
+ App._open_patch_in_external_editor. On success (launcher.launch_diff_result
  returns a process), returns Result(data=True).
  """
  from src import gui_2
@@ -1045,7 +1045,7 @@ def test_phase_5_l1393_open_patch_in_external_editor_result_success():
  mock_launcher    = MagicMock(name="mock_launcher")
  mock_launcher.config.get_default.return_value = mock_editor
  mock_process     = MagicMock(name="mock_process")
- mock_launcher.launch_diff.return_value         = mock_process
+ mock_launcher.launch_diff_result.return_value  = MagicMock(ok=True, data=mock_process)
  with patch("os.path.exists", return_value=True), \
    patch("src.external_editor.get_default_launcher", return_value=mock_launcher), \
    patch("src.external_editor.create_temp_modified_file", return_value="/tmp/patch_temp.py"):