Salvage of #14857 by @LeonSGP43 onto current main.

Closes #14850.

What this PR does

Stops hermes model and hermes doctor from printing false "Unknown provider" warnings for user-defined providers: entries in config.yaml. Before this PR, the validation/entrypoint paths called hermes_cli.auth.resolve_provider() (built-in providers only), while the runtime/picker paths correctly used resolve_provider_full() (user-aware). Same config, two answers.

How

• select_provider_and_model() in hermes_cli/main.py now routes through resolve_provider_full(effective_provider, user_providers, compatible_custom_providers) before falling back to resolve_provider('auto').
• run_doctor() in hermes_cli/doctor.py gets the same treatment plus extends its known_providers set with keys from providers: and custom_providers:.

Runtime behavior is unchanged — only validation/display paths were incorrectly strict.

Validation

With the issue's exact config (provider: volcengine-plan, providers.volcengine-plan.base_url: ark.cn-beijing.volces.com/api/coding/v3):

• tests/hermes_cli/test_doctor.py, test_setup.py — 41/41 pass (4 new regression tests)
• E2E: resolve_provider_full('volcengine-plan') returns ProviderDef(id='volcengine-plan', base_url='...', source='user-config')
• E2E: doctor run no longer emits the 'not a recognised provider' line

Co-authored-by: @LeonSGP43