fix(vision): preserve named custom providers

[haran2001]

Summary

• Preserve custom:<name> provider identifiers during auxiliary/vision provider normalization.
• Add a regression test that explicit named custom vision overrides route through resolve_provider_client() instead of the strict/default custom backend.

Test Plan

• python -m pytest tests/agent/test_auxiliary_main_first.py::TestResolveVisionMainFirst::test_named_custom_provider_override_preserves_custom_prefix -q -o 'addopts='

Closes #12638

[teknium1]

Thanks for the focused regression report and test.

Automated hermes-sweeper review found the requested routing behavior is already implemented on current main:

• agent/auxiliary_client.py:5286-5289 normalizes the explicit vision provider; named custom providers then take the generic _get_cached_client() route rather than a strict vision backend.
• agent/auxiliary_client.py:4435-4442 and :4715-4817 retain the pre-normalized identifier where needed and resolve the normalized named-provider suffix through _get_named_custom_provider().
• Commit 8ec1608642973fec9fcd90e962b11f9c18414247 deliberately removed the prior vision-specific custom-provider collapse; tests/agent/test_auxiliary_named_custom_providers.py:60-62 now guards the suffix-based behavior.
• The linked issue Withdrawn: vision provider routing report #12638 was withdrawn after this current-main behavior was identified.

Closing as implemented on main.