fix(gemini-cli): surface MODEL_CAPACITY_EXHAUSTED cleanly + drop retired gemma-4-26b (#11833)

Google-side 429 Code Assist errors now flow through Hermes' normal rate-limit
path (status_code on the exception, Retry-After preserved via error.response)
instead of being opaque RuntimeErrors. User sees a one-line capacity message
instead of a 500-char JSON dump.

Changes
- CodeAssistError grows status_code / response / retry_after / details attrs.
  _extract_status_code in error_classifier picks up status_code and classifies
  429 as FailoverReason.rate_limit, so fallback_providers triggers the same
  way it does for SDK errors. run_agent.py line ~10428 already walks
  error.response.headers for Retry-After — preserving the response means that
  path just works.
- _gemini_http_error parses the Google error envelope (error.status +
  error.details[].reason from google.rpc.ErrorInfo, retryDelay from
  google.rpc.RetryInfo). MODEL_CAPACITY_EXHAUSTED / RESOURCE_EXHAUSTED / 404
  model-not-found each produce a human-readable message; unknown shapes fall
  back to the previous raw-body format.
- Drop gemma-4-26b-it from hermes_cli/models.py, hermes_cli/setup.py, and
  agent/model_metadata.py — Google returned 404 for it today in local repro.
  Kept gemma-4-31b-it (capacity-constrained but not retired).

Validation
|                           | Before                         | After                                     |
|---------------------------|--------------------------------|-------------------------------------------|
| Error message             | 'Code Assist returned HTTP 429: {500 chars JSON}' | 'Gemini capacity exhausted for gemini-2.5-pro (Google-side throttle...)' |
| status_code on error      | None (opaque RuntimeError)     | 429                                       |
| Classifier reason         | unknown (string-match fallback) | FailoverReason.rate_limit                |
| Retry-After honored       | ignored                        | extracted from RetryInfo or header        |
| gemma-4-26b-it picker     | advertised (404s on Google)    | removed                                   |

Unit + E2E tests cover non-streaming 429, streaming 429, 404 model-not-found,
Retry-After header fallback, malformed body, and classifier integration.
Targeted suites: tests/agent/test_gemini_cloudcode.py (81 tests), full
tests/hermes_cli (2203 tests) green.

Co-authored-by: teknium1 <teknium@nousresearch.com>

agent/google_code_assist.py

@@ -68,9 +68,45 @@ _ONBOARDING_POLL_INTERVAL_SECONDS = 5.0
 
 
 class CodeAssistError(RuntimeError):
-    def __init__(self, message: str, *, code: str = "code_assist_error") -> None:
+    """Exception raised by the Code Assist (``cloudcode-pa``) integration.
+
+    Carries HTTP status / response / retry-after metadata so the agent's
+    ``error_classifier._extract_status_code`` and the main loop's Retry-After
+    handling (which walks ``error.response.headers``) pick up the right
+    signals.  Without these, 429s from the OAuth path look like opaque
+    ``RuntimeError`` and skip the rate-limit path.
+    """
+
+    def __init__(
+        self,
+        message: str,
+        *,
+        code: str = "code_assist_error",
+        status_code: Optional[int] = None,
+        response: Any = None,
+        retry_after: Optional[float] = None,
+        details: Optional[Dict[str, Any]] = None,
+    ) -> None:
         super().__init__(message)
         self.code = code
+        # ``status_code`` is picked up by ``agent.error_classifier._extract_status_code``
+        # so a 429 from Code Assist classifies as FailoverReason.rate_limit and
+        # triggers the main loop's fallback_providers chain the same way SDK
+        # errors do.
+        self.status_code = status_code
+        # ``response`` is the underlying ``httpx.Response`` (or a shim with a
+        # ``.headers`` mapping and ``.json()`` method).  The main loop reads
+        # ``error.response.headers["Retry-After"]`` to honor Google's retry
+        # hints when the backend throttles us.
+        self.response = response
+        # Parsed ``Retry-After`` seconds (kept separately for convenience —
+        # Google returns retry hints in both the header and the error body's
+        # ``google.rpc.RetryInfo`` details, and we pick whichever we found).
+        self.retry_after = retry_after
+        # Parsed structured error details from the Google error envelope
+        # (e.g. ``{"reason": "MODEL_CAPACITY_EXHAUSTED", "status": "RESOURCE_EXHAUSTED"}``).
+        # Useful for logging and for tests that want to assert on specifics.
+        self.details = details or {}
 
 
 class ProjectIdRequiredError(CodeAssistError):