OpenRouter provider hardcodes reasoning efforts in copilot extension

Does this issue occur when all extensions are disabled?: Yes

- VS Code Version: 1.134.0
- OS Version: VSCode web in GH Codespaces

Steps to Reproduce:

1. Enroll an OpenRouter API key via the `Language Models` dialog, with the `OpenRouter` provider.

2. Pick a model that supports a reasoning effort `xhigh` (e.g., `OpenAI: GPT-5.6 Luna`).

3. Try to select the `xhigh` reasoning effort: it won't be available.

The reason is very clearly in these lines:
https://github.com/microsoft/vscode/blob/f3fa55c39d3df2923b46a3d76cf6baf0afa1db33/extensions/copilot/src/extension/byok/vscode-node/openRouterProvider.ts#L90-L92

And the OpenRouter API does provide this information:
https://openrouter.ai/docs/api/api-reference/models/list-all-models-and-their-properties#response-data-items-reasoning

**I can try to submit a PR for this. I'm opening this first for maintainers to confirm this can be changed, and that it won't have unintended consequences.**