fix(image_routing): check main model vision support before aux override

[blut-agent]

Bug

When auxiliary.vision is explicitly configured, images were forced through the text pipeline even when the main model supports vision (e.g. xiaomi/mimo-v2.5).

Root Cause

In agent/image_routing.py, decide_image_input_mode() checked _explicit_aux_vision_override() before checking main model vision support, unconditionally blocking the native path.

Fix

Reorder checks: main model vision support is checked first, aux override becomes a fallback for non-vision models only.

Changes

• agent/image_routing.py: reordered checks in decide_image_input_mode(), updated module docstring
• tests/agent/test_image_routing.py: split test into two — verifies native when main model supports vision (with aux configured), and text when main model is non-vision (with aux configured)

Tests

• tests/agent/test_image_routing.py: 76 passed
• tests/agent/test_custom_providers_vision.py: 13 passed

[liuhao1024]

✅ Verified — vision routing priority reorder

Reviewed the diff for decide_image_input_mode priority logic in agent/image_routing.py.

• Priority inversion is correct: The main model's supports_vision check now runs before _explicit_aux_vision_override. Previously, an explicit aux backend would override even a vision-capable main model — forcing unnecessary round-trips through the text pipeline. The new order (native if main sees → text with aux if configured → text fallback) matches the intended "main model first, aux as fallback" semantics.
• Test coverage is thorough: test_explicit_aux_vision_override_only_applies_when_main_model_not_vision verifies the new priority (native wins over aux), and test_aux_vision_override_wins_when_main_model_not_vision confirms aux still activates for non-vision models. Both paths are covered.
• Docstring updated: The module docstring correctly reflects the new priority order.

No issues found. The reorder is a clean semantic improvement.

[alt-glitch]

Duplicate of #32519 (and the earlier #29138 lineage) — same fix in agent/image_routing.py: check main-model vision support before the auxiliary.vision override in decide_image_input_mode() so aux acts as a true fallback only for non-vision models. Also implements the bug half of #44299.

[blut-agent]

Closing — duplicate of earlier PR #45651 or #33699/#32519. See the canonical PR for tracking.