fix: cap vision image pixel dimensions at 8000px to prevent Anthropic 400 errors

[zccyman]

Problem

Anthropic's Messages API rejects any image larger than 8000 px on either axis with a non-retryable 400 error. Once such an image enters the message history, every subsequent request fails — the session is permanently bricked.

Fix

Add a pixel dimension check (_MAX_PIXEL_DIMENSION = 8000) in two places:

1. _vision_analyze_native() — BEFORE base64 encoding, so oversized-but-small-byte-count images are caught early
2. _resize_image_for_vision() — as an additional safeguard for existing callers that go through the resize path

Both checks:

• Use Pillow (Image.LANCZOS) to resize while preserving aspect ratio
• Gracefully degrade if Pillow is not installed (log + skip)

Testing

• 6 unit tests + 1 skip (no Pillow): all pass
• Tests cover: oversized width, oversized height, both axes, boundary (8000px exact), aspect ratio preservation, graceful Pillow absence

Closes #25837

[alt-glitch]

Duplicate of #25838 — both cap vision image pixel dimensions to prevent Anthropic 400 errors on oversized images. Both fix #25837.

[zccyman]

Thanks @alt-glitch for the catch. Duplicate of #25838 — both cap vision image pixel dimensions at 8000px. Closing in favor of #25838.