fix(cli): treat named custom providers as aggregators

[googlerest]

Summary

• Treat named custom providers like custom:zenmux as aggregators for model alias resolution.
• Suppress the false-positive doctor vendor-prefix warning for named custom providers.
• Add regressions for is_aggregator("custom:zenmux") and hermes doctor with custom:zenmux + z-ai/glm-5.1.

Fixes #26578.

Verification

• scripts/run_tests.sh tests/hermes_cli/test_model_switch_custom_providers.py::test_named_custom_provider_is_aggregator tests/hermes_cli/test_doctor.py::test_run_doctor_accepts_vendor_slug_for_named_custom_provider
• scripts/run_tests.sh tests/hermes_cli/test_model_switch_custom_providers.py tests/hermes_cli/test_doctor.py
• git diff --check
• python -m py_compile hermes_cli/providers.py hermes_cli/doctor.py

[alt-glitch]

Duplicate of #26594 — both PRs fix the same issue (#26578: custom:<name> providers not recognized as aggregators). Both make identical functional changes to providers.py and doctor.py. #26594 has more thorough docstrings and test coverage.

[googlerest]

Duplicate of #26594 which covers the same fix. Closing in favor of that PR.