feat: restore Unbound as a provider

[pugazhendhi-m]

Related GitHub Issue

N/A - Restoring provider that was removed in #11297

Description

Restores Unbound as a dynamic provider after it was removed in #11297.

Changes:

• Added Unbound type definitions, API handler (UnboundHandler), and model fetcher
• Added Unbound to the provider settings schema with unboundApiKey and unboundModelId
• Added UI component matching Requesty's pattern (API key input, refresh models button, model picker)
• Added Unbound in alphabetical order in the Providers dropdown
• Added translations for all 17 locales
• Updated all affected test files

Test Procedure

1. Select "Unbound" from the providers dropdown — verify it appears in alphabetical order
2. Enter an Unbound API key — verify it saves
3. Click "Refresh Models" — verify models are fetched from the API
4. Select a model and send a message — verify it works
5. Run pnpm test — all tests pass (except pre-existing CustomModesManager.yamlEdgeCases failure)

Pre-Submission Checklist

• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes (if applicable).
• Documentation Impact: I have considered if my changes require documentation updates (see "Documentation Updates" section below).
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Screenshots / Videos

N/A

Documentation Updates

No documentation updates required.

Additional Notes

• The UnboundHandler follows the same OpenAI SDK pattern used by other providers
• Refresh models button uses vscode.postMessage with requestRouterModels matching Requesty's exact pattern
• All 17 locale translation files updated with unboundApiKey and getUnboundApiKey keys