fix(vision): guard image pixel dimensions, not just bytes (#37677)

Anthropic enforces two independent ceilings per image:
1. 5 MB encoded byte size
2. 8000 px longest side

Hermes only guarded #1. A tall screenshot (e.g. 1200x12000 at 0.06 MB)
passes every byte check but fails the pixel check, returning a
non-retryable HTTP 400 that permanently bricks the conversation thread.

Fixes:
- error_classifier: add 'image dimensions exceed' pattern to
  _IMAGE_TOO_LARGE_PATTERNS so the 400 is classified as image_too_large
  and triggers the shrink/retry path instead of falling through to
  non-retryable error.
- conversation_compression: check pixel dimensions (via Pillow) even
  when byte size is under the 4 MB target. If max(dims) > 8000, force
  shrink.
- vision_tools._resize_image_for_vision: add optional max_dimension param.
  When set, images exceeding the pixel cap are downscaled even if they're
  under the byte budget. The resize loop now checks both byte AND pixel
  limits before accepting a candidate.

Closes #37677

tests/run_agent/test_image_shrink_recovery.py

@@ -143,7 +143,7 @@ class TestShrinkImagePartsHelper:
         oversized_url = _big_png_data_url(5000)  # ~5 MB raw → ~6.7 MB b64
         shrunk = "data:image/jpeg;base64," + "A" * 1000  # small
 
-        def _fake_resize(path, mime_type=None, max_base64_bytes=None):
+        def _fake_resize(path, mime_type=None, max_base64_bytes=None, max_dimension=None):
             return shrunk
 
         monkeypatch.setattr(