refactor(ai_client): migrate set_tool_preset + set_bias_profile to Result[T] (Phase 11 sites 5+6)

Both functions had:
  try: ToolPresetManager().load_all() ...
  except (OSError, ValueError, AttributeError) as e:
      sys.stderr.write(f'[ERROR] Failed to set {preset_name}: {e}')
      sys.stderr.flush()

sys.stderr.write is logging = NOT a drain = SS violation per MUST-NOT-DO #6.

New helpers:
- _set_tool_preset_result(preset_name: Optional[str]) -> Result[None]
  Empty/None preset short-circuits to Result(data=None).
  On failure: Result(data=None, errors=[ErrorInfo]).
- _set_bias_profile_result(profile_name: Optional[str]) -> Result[None]
  Same pattern.

Legacy wrappers set the global state (or skip on empty preset) and
delegate to the _result helper. Cache invalidation runs regardless.

Audit: ai_client SS 9 -> 7. COMPLIANT 27 -> 29.

tests/tier2/phase11_sites56_test.py

@@ -0,0 +1,41 @@
+"""Phase 11 sites 5+6: set_tool_preset + set_bias_profile Result helpers.
+
+Both had:
+    try: ToolPresetManager().load_all() ...
+    except (OSError, ValueError, AttributeError) as e:
+        sys.stderr.write(f'[ERROR] Failed to set {preset_name}: {e}')
+        sys.stderr.flush()
+
+Body is sys.stderr.write = logging NOT a drain = SS violation.
+MIGRATE to Result[None].
+"""
+import sys
+sys.path.insert(0, ".")
+
+
+def test_phase11_sites56_set_tool_preset_result_exists():
+    import src.ai_client
+    assert hasattr(src.ai_client, "_set_tool_preset_result"), \
+        "_set_tool_preset_result helper missing"
+
+
+def test_phase11_sites56_set_bias_profile_result_exists():
+    import src.ai_client
+    assert hasattr(src.ai_client, "_set_bias_profile_result"), \
+        "_set_bias_profile_result helper missing"
+
+
+def test_phase11_sites56_helpers_return_result():
+    import src.ai_client
+    import inspect
+    for name in ("_set_tool_preset_result", "_set_bias_profile_result"):
+        fn = getattr(src.ai_client, name)
+        sig = inspect.signature(fn)
+        assert "Result" in str(sig.return_annotation), \
+            f"{name} return must be Result, got {sig.return_annotation}"
+
+
+def test_phase11_sites56_legacy_preserved():
+    import src.ai_client
+    assert callable(getattr(src.ai_client, "set_tool_preset", None))
+    assert callable(getattr(src.ai_client, "set_bias_profile", None))