fix(image-routing): check native vision support before aux override

[liuhao1024]

Problem

When auxiliary.vision is explicitly configured in config.yaml, _explicit_aux_vision_override() forces all images through the auxiliary vision LLM text pipeline — even when the main model itself supports vision natively (e.g. mimo-v2.5 with supports_vision=True).

This causes two issues:

1. Unnecessary latency — images are sent to an auxiliary vision model instead of being handled directly by the main model
2. Provider failures — providers like Xiaomi MiMo that accept images in user messages but reject them in tool_result messages get HTTP 400 errors when images are routed through the auxiliary pipeline

Root Cause

In agent/image_routing.py, decide_image_input_mode() checks _explicit_aux_vision_override() before checking native vision support:

# Before (buggy)
if _explicit_aux_vision_override(cfg):
    return "text"  # Always, regardless of main model capabilities

supports = _lookup_supports_vision(provider, model, cfg)
if supports is True:
    return "native"  # Never reached when aux is configured

Fix

Swap the check order: verify native vision support first, then fall through to aux override only when the main model does NOT support vision:

# After (fixed)
supports = _lookup_supports_vision(provider, model, cfg)
if supports is True:
    return "native"  # Main model can see images → use it

if _explicit_aux_vision_override(cfg):
    return "text"  # Main model can't see → use aux if configured

Changes

• agent/image_routing.py — Swap check order in decide_image_input_mode()
• tests/agent/test_image_routing.py — Update 2 tests to reflect new priority (native wins over aux), add 1 test for aux fallback when main model lacks vision

Testing

77 passed in 0.35s (test_image_routing.py)
30 passed in 0.23s (test_computer_use_vision_routing.py)

Closes #44299

[alt-glitch]

Duplicate of #44305 — same reorder of decide_image_input_mode() (check native vision support before _explicit_aux_vision_override), both implementing the bug half of #44299 and descending from #32519/#29135. #44305 was opened earlier; consolidating there.

[liuhao1024]

Thanks for the flag @alt-glitch. Comparing the two PRs:

• fix(image_routing): check main model vision support before aux override #44305: Reorders decide_image_input_mode() to check native vision first + updates docstring + 2 tests
• fix(image-routing): check native vision support before aux override #44320 (this PR): Same reorder + 3 tests (covers native-vision-wins, aux-override-as-fallback, and config-level supports_vision)

Both implement the same fix for #44299. The code changes are equivalent; this PR adds slightly more test coverage. Happy to close if #44305 is preferred.

[liuhao1024]

Thanks for the flag @alt-glitch. Confirmed — #44305 has the same reorder (check native vision before aux override) and was opened earlier the same day. Closing in favor of #44305.