feat: add Copilot reasoning effort picker support for OpenRouter

[nicolalazzaro]

Summary

This PR extends the Copilot Reasoning Effort picker support to OpenRouter-compatible models.

Following the implementation in #237 for OpenAI-compatible models, this PR allows users to temporarily override the reasoning effort for a single request when using OpenRouter models.

Behavior

• If reasoning.effort is unset or empty:

  • the model is registered normally;
  • no Copilot reasoning effort picker is shown.

• If reasoning.effort is set to a supported value:

  • the model is registered with a Copilot configurationSchema;
  • the Copilot chat input shows a Reasoning Effort picker;
  • the configured reasoning.effort value is used as the picker default.

• If the user changes the picker value in Copilot:

  • that temporary selection is used for the current request.

• If Copilot does not pass a temporary picker value:

  • the configured model-level reasoning.effort is used as the fallback.

Implementation

• Added isOpenRouterReasoningEffortPickerEnabled helper in src/modelConfiguration.ts.
• Updated prepareLanguageModelChatInformation in src/provideModel.ts to register the configurationSchema for OpenRouter models.
• Updated OpenaiApi in src/openai/openaiApi.ts to apply the selected reasoning effort to the reasoning.effort field in the OpenRouter request body.
• Updated the configuration UI in assets/configView/configView.html to clarify that leaving the Reasoning Effort field empty hides the Copilot picker.
• Added tests in src/test/modelConfiguration.test.ts to verify the new functionality.