fix(media): normalize ICO images to PNG

[lihailong00]

Summary

• normalize ICO image payloads to PNG before sending them to the model
• preserve the original MIME type, byte length, and dimensions in the user-facing metadata
• leave all other image formats unchanged

Reproduction and verification

Before the fix, the regression test deterministically produced a data:image/x-icon;base64 model payload. After the fix:

• uv run pytest tests/tools/test_read_media_file.py -q — 7 passed
• targeted Ruff check — passed
• targeted Pyright — 0 errors

Closes #2367

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.