Fix accessibility label for Copilot model picker dropdown

[Copilot]

The Copilot model picker dropdown in inline chat (Ctrl+I) was not providing adequate accessibility labels for screen reader users. Previously, the dropdown only announced "Pick Model - {model_name}" which didn't clearly communicate the purpose of the control.

Changes Made

1. Updated tooltip text: Changed from "Pick Model" to "Select Language Model" to provide clearer context
2. Enhanced aria-label: Added custom setAriaLabelAttributes method that provides a descriptive label: "Select language model, currently {model_name}"

Before vs After

Before: Screen readers announced "Pick Model - GPT-4"
After: Screen readers announce "Select language model, currently GPT-4"

Technical Details

• Overrides setAriaLabelAttributes in ModelPickerActionItem class
• Uses VS Code's localize() function for internationalization support
• Maintains all existing functionality while improving accessibility
• Automatically updates when the model selection changes

The fix ensures compliance with WCAG 4.1.2 (Name, Role, Value) by providing clear, descriptive labels that help screen reader users understand both the control's purpose and current state.

Fixes #250598.

Warning

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