Add confidence score or extended metadata support to AITextSearchProvider

## Feature Request
Enhance the `AITextSearchProvider` and related types in `vscode.d.ts` to support optional metadata such as confidence scores or similarity metrics for AI-generated search results.

### Problem or Use Case
Currently, `provideAITextSearchResults` returns `TextSearchResult2` via progress callbacks, but there is no native way to pass AI-specific metrics like a confidence score or relevance weight per result. This limits extensions trying to sort or display advanced AI search telemetry in the Search View.

### Proposed Solution
Add optional properties (e.g., confidence score or extended options) to the relevant interfaces to improve extensibility for AI search providers.