fix: discover models from configured providers

[Asunfly]

Summary

• Discover available model groups from Hermes Agent's newer config.providers.<name> entries.
• Merge configured provider models with existing provider groups instead of dropping models when provider keys overlap.
• Support default model fallbacks from default_model, model.default, and model.model.
• Validate key_env names before reading environment variables.

Why

Hermes Agent can load configurations where custom providers are declared under providers, for example:

model:
  default: custom-model-pro
  provider: customrelay
providers:
  customrelay:
    key_env: CUSTOM_RELAY_API_KEY
    default_model: custom-model-pro
    models:
      custom-model-pro: {}

The server config endpoint could read this shape, but /api/hermes/available-models did not expose the configured provider/model group. That left the model selector empty even though the active Hermes config was valid.

Duplicate check

I checked existing related issues/PRs before opening this:

• Improve available-models: dynamic /models discovery for all built-in providers with static fallback #822 provider dynamic model discovery — related, but focused on provider APIs/built-ins.
• [codex] fix profile scoped model selection #881 profile-scoped model selection — related UI/model selection work.
• feat: update FUN providers, add builtin flag, api_mode, and API relay link #522 provider metadata/API relay — related provider metadata topic.
• feat: support manual model input and sync provider catalogs #34 custom provider manual model input — related custom provider workflow.

I did not find an existing issue/PR specifically covering config.providers.<name> discovery in /api/hermes/available-models.

Test plan

Local verification performed:

• git diff --check
• npx vitest run tests/server/model-visibility-controller.test.ts tests/server/config-helpers-file-lock.test.ts tests/server/health-controller.test.ts tests/client/models-store.test.ts
  • 4 test files passed
  • 25 tests passed
• npm run build

Live verification against a local service using the newer Hermes config shape:

• /health returned HTTP 200
• /api/hermes/profiles returned HTTP 200
• /api/hermes/available-models returned the configured custom provider group and its default model.