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)

src/app_controller.py

@@ -4211,7 +4211,7 @@ class AppController:
    stream_callback=lambda text: self._on_ai_stream(text),
    pre_tool_callback=self._confirm_and_run,
    qa_callback=ai_client.run_tier4_analysis,
-   patch_callback=ai_client.run_tier4_patch_callback,
+   patch_callback=ai_client._run_tier4_patch_callback_result,
    rag_engine=None, # Already handled above
   )
   if result.ok:
@@ -4227,8 +4227,8 @@ class AppController:
   [C: tests/test_app_controller_offloading.py:test_on_tool_log_offloading]
   """
   session_logger.log_tool_call(script, result, None)
-  session_logger.log_tool_output(result)
-  source_tier = ai_client.get_current_tier()
+  session_logger.log_tool_output_result(result)
+  source_tier = ai_client.get_current_tier_result().data
   with self._pending_tool_calls_lock:
    self._pending_tool_calls.append({"script": script, "result": result, "ts": time.time(), "source_tier": source_tier})
 
@@ -4238,9 +4238,9 @@ class AppController:
   payload = optimized.get("payload", {})
   if kind == "tool_result" and "output" in payload:
    output = payload["output"]
-   ref_path = session_logger.log_tool_output(output)
-   if ref_path:
-    filename = Path(ref_path).name
+   ref_result = session_logger.log_tool_output_result(output)
+   if ref_result.ok and ref_result.data:
+    filename = Path(ref_result.data).name
     payload["output"] = f"[REF:{filename}]"
   if kind == "tool_call" and "script" in payload:
    script = payload["script"]
@@ -4394,7 +4394,7 @@ class AppController:
   if self.ui_auto_scroll_tool_calls:
    self._scroll_tool_calls_to_bottom = True
 
- def _confirm_and_run(self, script: str, base_dir: str, qa_callback: Optional[Callable[[str], str]] = None, patch_callback: Optional[Callable[[str, str], Optional[str]]] = None) -> Optional[str]:
+ def _confirm_and_run(self, script: str, base_dir: str, qa_callback: Optional[Callable[[str], str]] = None, patch_callback: Optional[Callable[[str, str], Result[str]]] = None) -> Optional[str]:
   """
     [C: tests/test_arch_boundary_phase2.py:TestArchBoundaryPhase2.test_mutating_tool_triggers_callback, tests/test_arch_boundary_phase2.py:TestArchBoundaryPhase2.test_rejection_prevents_dispatch]
   """