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

agent/error_classifier.py

@@ -171,6 +171,7 @@ _IMAGE_TOO_LARGE_PATTERNS = [
     "image too large",      # generic
     "image_too_large",      # error_code variant
     "image size exceeds",   # variant
+    "image dimensions exceed",  # Anthropic: "image dimensions exceed max allowed size: 8000 pixels"
     # "request_too_large" on a request known to contain an image → image is
     # the likely culprit; we still try the shrink path before giving up.
 ]