fix: shrink images to provider dimension limit

[OutThisLife]

Summary

• Parse provider-reported image dimension ceilings from image-too-large errors.
• Pass the parsed ceiling into image shrink recovery so many-image Anthropic requests can downscale Retina screenshots below 2000px before retrying.
• Add regression coverage for the OpenRouter-wrapped Anthropic many-image rejection.

Test plan

• scripts/run_tests.sh tests/run_agent/test_image_shrink_recovery.py
• scripts/run_tests.sh tests/run_agent/test_image_shrink_recovery.py tests/run_agent/test_image_rejection_fallback.py tests/run_agent/test_multimodal_tool_content_recovery.py
• ReadLints on edited files
• git diff --check

[github-actions]

🔎 Lint report: bb/image-shrink-many-image vs origin/main

ruff

Total: 0 on HEAD, 0 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 0 pre-existing issues carried over.

ty (type checker)

Total: 10889 on HEAD, 10889 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 5724 pre-existing issues carried over.

Diagnostics are surfaced as warnings — this check never fails the build.

[liuhao1024]

Verification: Reviewed the full diff across 4 files. The _image_error_max_dimension() extraction from provider error messages is well-scoped — the regex anchors on "max allowed size" + "pixels" which limits false matches, and the 512–8000 sanity range prevents garbage values from passing through. The max_parameter threading through try_shrink_image_parts_in_messages → _resize_image_for_vision is clean with a safe 8000 fallback. Tests cover the OpenRouter-wrapped Anthropic many-image case and verify the dimension propagates correctly to the resize call. No issues found.