fix(agent): apply the oversize content-field rule on the 422 path too

pydantic relays (#104731) report the same content-field detail with HTTP 422; the 422 handler
ran only the keyword rules, so a Nebius-shaped rejection behind such a relay would still be
claimed by the multimodal tool-content rule. Sibling site of #112473.

agent/error_classifier.py

@@ -916,6 +916,13 @@ def _classify_400(c: _Ctx) -> Verdict:
     return _V_FORMAT_ERROR
 
 
+def _classify_image_tool_422(c: _Ctx) -> Verdict:
+    """422: pydantic relays report the same content-field shapes as 400 (#104731, #112473)."""
+    if _oversized_message_content_rejection(c.body):
+        return _V_IMAGE_TOO_LARGE
+    return _first_match(c.msg, _IMAGE_TOOL_RULES) or _V_FORMAT_ERROR
+
+
 # 401 not retryable on its own: rotation/refresh run before the retryability
 # check, then the client-error abort path (fallback first) is correct. 408 is
 # retry-safe (RFC 9110 §15.5.9; proxies emit it when generation outruns the
@@ -923,7 +930,7 @@ def _classify_400(c: _Ctx) -> Verdict:
 _STATUS_HANDLERS: Dict[int, Callable[[_Ctx], Verdict]] = {
     400: _classify_400, 401: lambda c: _V_AUTH_ROTATE, 402: lambda c: _classify_402(c.msg, dict),
     403: _status_403, 404: _status_404, 408: lambda c: _V_TIMEOUT, 413: lambda c: _V_PAYLOAD_TOO_LARGE,
-    422: lambda c: _first_match(c.msg, _IMAGE_TOOL_RULES) or _V_FORMAT_ERROR,
+    422: lambda c: _classify_image_tool_422(c),
     429: _status_429, 500: _status_5xx, 502: _status_5xx,
     503: lambda c: _first_match(c.msg, _OVERFLOW_AS_5XX_RULES) or _V_OVERLOADED,
     529: lambda c: _first_match(c.msg, _OVERFLOW_AS_5XX_RULES) or _V_OVERLOADED,