refactor(ai_client): migrate L1594 _list_gemini_models to Result[T] (Phase 10 site 1)

The original function had a broken pattern: 'raise _classify_gemini_error(exc)
from exc' which raises an ErrorInfo (not an Exception) — a runtime bug.

Per TIER1_REVIEW 2026-06-20 directive: per-site decision. The body raised a
structured error carrier (ErrorInfo), but the pattern was incorrect (ErrorInfo
is not an Exception). Cleanest fix: full Result[T] migration.

New helper:
- _list_gemini_models_result(api_key: str) -> Result[list[str]]
  Returns Result(data=sorted_models) on success, Result(data=[], errors=[ErrorInfo])
  on SDK/network failure.

Legacy wrapper:
- _list_gemini_models(api_key: str) -> list[str]
  Returns result.data (preserves original signature; callers don't see errors).

Audit: ai_client BC 9 -> 8. Site L1594 (now shifted to L1609 due to helper insertion)
no longer in INTERNAL_BROAD_CATCH.

src/ai_client.py

@@ -1581,7 +1581,15 @@ def _list_gemini_cli_models() -> list[str]:
   "gemini-2.5-flash-lite",
  ]
 
-def _list_gemini_models(api_key: str) -> list[str]:
+def _list_gemini_models_result(api_key: str) -> Result[list[str]]:
+ """List available Gemini models via google-genai SDK.
+
+ Returns the sorted list of Gemini model names. On SDK or network failure,
+ returns Result(data=[], errors=[ErrorInfo(...)]). The legacy caller
+ (_list_gemini_models) returns result.data directly (preserving original
+ behavior); callers that need to surface errors should call this helper
+ and inspect result.errors.
+ """
  try:
   genai  = _require_warmed("google.genai")
   client = genai.Client(api_key=api_key)
@@ -1590,9 +1598,16 @@ def _list_gemini_models(api_key: str) -> list[str]:
    name = m.name
    if name and name.startswith("models/"): name = name[len("models/"):]
    if name and "gemini" in name.lower():   models.append(name)
-  return sorted(models)
+  return Result(data=sorted(models))
  except Exception as exc:
-  raise _classify_gemini_error(exc) from exc
+  return Result(
+   data=[],
+   errors=[_classify_gemini_error(exc, source="ai_client._list_gemini_models_result")],
+  )
+
+
+def _list_gemini_models(api_key: str) -> list[str]:
+ return _list_gemini_models_result(api_key).data
 
 def _ensure_gemini_client() -> None:
   global _gemini_client