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/tier2/phase10_invariant_test.py

@@ -48,22 +48,22 @@ def test_phase10_all_helpers_exist():
 
 
 def test_phase10_legacy_functions_preserved():
-    """Legacy functions preserved EXCEPT those OBLITERATED by cruft-removal Phase 4."""
+    """Legacy functions preserved EXCEPT those OBLITERATED by cruft-removal Phase 4 or code_path_audit_phase_2 cleanup."""
     import src.ai_client
     legacy = [
         "_send_gemini",
         "_send_gemini_cli",
         "run_tier4_analysis",
-        "run_tier4_patch_callback",
         "run_tier4_patch_generation",
     ]
     # _list_gemini_models wrapper was OBLITERATED by cruft-removal Phase 4
-    obliterated = ["_list_gemini_models"]
+    # run_tier4_patch_callback wrapper was OBLITERATED by code_path_audit_phase_2 cleanup
+    obliterated = ["_list_gemini_models", "run_tier4_patch_callback"]
     for name in legacy:
         assert hasattr(src.ai_client, name), f"{name} legacy function missing"
         assert callable(getattr(src.ai_client, name)), f"{name} not callable"
     for name in obliterated:
         assert not hasattr(src.ai_client, name), (
-            f"{name} wrapper must be OBLITERATED (cruft-removal Phase 4); "
+            f"{name} wrapper must be OBLITERATED; "
             f"callers must use {name}_result directly"
         )

tests/tier2/phase10_sites789_test.py

@@ -45,10 +45,23 @@ def test_phase10_sites789_all_helpers_return_result():
 
 
 def test_phase10_sites789_legacy_unchanged():
-    """Legacy functions must still exist + be callable."""
+    """Legacy functions preserved EXCEPT those OBLITERATED by code_path_audit_phase_2 cleanup.
+
+    run_tier4_patch_callback was a T|None wrapper (heuristic bypass per review Finding 8)
+    whose only consumers were callback references in app_controller.py and
+    multi_agent_conductor.py. After this cleanup track:
+    - The callback contract migrated to Callable[[str, str], Result[str]]
+    - The 2 callers now pass _run_tier4_patch_callback_result directly
+    - run_tier4_patch_callback wrapper is gone
+    """
     import src.ai_client
-    for name in ("run_tier4_analysis",
-                 "run_tier4_patch_callback",
-                 "run_tier4_patch_generation"):
+    legacy = ["run_tier4_analysis", "run_tier4_patch_generation"]
+    obliterated = ["run_tier4_patch_callback"]
+    for name in legacy:
         assert hasattr(src.ai_client, name), f"{name} missing"
-        assert callable(getattr(src.ai_client, name)), f"{name} not callable"
+        assert callable(getattr(src.ai_client, name)), f"{name} not callable"
+    for name in obliterated:
+        assert not hasattr(src.ai_client, name), (
+            f"{name} wrapper must be OBLITERATED (code_path_audit_phase_2 cleanup); "
+            f"callers must use {name}_result directly"
+        )