fix(auxiliary): catch ImportError from build_anthropic_client in vision auto-detection (#3312)

_try_anthropic() caught ImportError on the module import (line 667-669)
but not on the build_anthropic_client() call (line 696). When the
anthropic_adapter module imports fine but the anthropic SDK is missing,
build_anthropic_client() raises ImportError at call time. This escaped
_try_anthropic() entirely, killing get_available_vision_backends() and
cascading to 7 test failures:

- 4 setup wizard tests hit unexpected 'Configure vision:' prompt
- 3 codex-auth-as-vision tests failed check_vision_requirements()

The fix wraps the build_anthropic_client call in try/except ImportError,
returning (None, None) when the SDK is unavailable — consistent with the
existing guard at the top of the function.

agent/auxiliary_client.py

@@ -693,7 +693,13 @@ def _try_anthropic() -> Tuple[Optional[Any], Optional[str]]:
     is_oauth = _is_oauth_token(token)
     model = _API_KEY_PROVIDER_AUX_MODELS.get("anthropic", "claude-haiku-4-5-20251001")
     logger.debug("Auxiliary client: Anthropic native (%s) at %s (oauth=%s)", model, base_url, is_oauth)
-    real_client = build_anthropic_client(token, base_url)
+    try:
+        real_client = build_anthropic_client(token, base_url)
+    except ImportError:
+        # The anthropic_adapter module imports fine but the SDK itself is
+        # missing — build_anthropic_client raises ImportError at call time
+        # when _anthropic_sdk is None.  Treat as unavailable.
+        return None, None
     return AnthropicAuxiliaryClient(real_client, model, token, base_url, is_oauth=is_oauth), model