"No provider registered" log spam

## Reproduction

1. Launch Code OSS from sources with an isolated user-data directory and a fresh extensions directory.
2. Open the renderer process output log after startup.

Tested with Code OSS 1.129.0 at `db58c8918ca2ddba2b984f7f18fca7316023d8cb` on macOS.

## Actual behavior

Startup logs 72 warnings in under one second:

```text
[warning] [LM] No provider registered for vendor anthropic
[warning] [LM] No provider registered for vendor xai
[warning] [LM] No provider registered for vendor gemini
[warning] [LM] No provider registered for vendor openrouter
[warning] [LM] No provider registered for vendor openai
[warning] [LM] No provider registered for vendor ollama
[warning] [LM] No provider registered for vendor customoai
[warning] [LM] No provider registered for vendor customendpoint
[warning] [LM] No provider registered for vendor azure
```

Each of the nine vendor warnings is emitted eight times. These vendors are contributed by the built-in Copilot extension, but their providers are not registered during these startup queries.

The warning comes from `LanguageModelsService._resolveAllLanguageModels`. `selectLanguageModels` resolves every contributed vendor when the selector does not specify one, and repeated silent resolutions each log the same missing-provider warning. A missing provider appears to be an expected state for these optional providers, at least during startup.

## Expected behavior

Normal startup should not flood the renderer log with warnings for optional language model providers. Missing-provider diagnostics should either be deduplicated, emitted at a lower level for silent resolution, or avoided until a provider is expected to exist.

Related: [#264241](https://github.com/microsoft/vscode/issues/264241) contains the same repeated warning in a test-failure context, but does not track the normal Code OSS startup log noise.

(Written by Copilot)