fix: send a single auth header when listing models and testing keys

[MFA-G]

Problem

For an OpenAI-compatible custom provider, Jan sends both authentication headers when testing an API key and refreshing the model list:

Authorization: *** <key>
x-api-key: <key>

Some upstreams reject requests that carry both. AWS Bedrock Mantle answers 401 Unauthorized:

{ "error": { "code": "invalid_api_key",
  "message": "request must not include both 'authorization' and 'x-api-key' headers" } }

So a valid key fails Test keys and model refresh, even though normal chat requests work (they already send only the bearer header).

Fixes #8444.

Fix

Add applyProviderAuthHeader() in remoteModelCatalog.ts and use it in the three request paths that previously hard-coded both headers:

• TauriProvidersService.fetchModelsFromProvider (services/providers/tauri.ts)
• fetchTopRemoteModels → buildHeaders (lib/remoteModelCatalog.ts)
• handleTestApiKeys (routes/settings/providers/$providerName.tsx)

Behavior now matches how chat requests already authenticate in model-factory.ts:

• Anthropic-shaped providers (api_type === 'anthropic', or name/host contains anthropic) → x-api-key
• Everything else (OpenAI-compatible) → Authorization: *** <key>

The key-test path additionally gains the default anthropic-version / browser-access headers (via the existing ensureAnthropicHeaders), so an Anthropic key test behaves the same as a model refresh.

Tests

Updated/added unit tests asserting exactly one auth header per provider type:

• lib/__tests__/remoteModelCatalog.test.ts — OpenAI sends only Authorization (no x-api-key); Anthropic sends only x-api-key (no Authorization).
• services/providers/__tests__/tauri.test.ts — same, plus an anthropic-shaped custom provider case.

Validation:

• yarn vitest --run on the three affected files: 79 tests pass.
• Full web-app suite: 2786 tests pass (the one unrelated failing file, services/core/__tests__/tauri.test.ts, fails only because the @janhq/assistant-extension workspace package isn't built in this environment — unrelated to this change).
• yarn eslint on all changed source files: clean.