fix: pass providerType to chat api

[ChaseAfterLight]

Summary

Fixes chat requests for custom providers by passing providerType from the client to /api/chat and through server model resolution.

Related Issues

None.

Changes

• Added providerType to the stateless chat request type.
• Passed providerType from use-chat-sessions when sending chat requests.
• Forwarded providerType into getModel() in /api/chat.
• Keeps custom providers from failing with Unknown provider ... Please provide providerType.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• CI/CD or build changes

Verification

Steps to reproduce / test

1. Configure a custom provider.
2. Send a chat message through the chat session flow.
3. Confirm the request no longer fails with Unknown provider ... Please provide providerType.

What you personally verified

• Confirmed the code path that builds /api/chat requests now includes providerType.
• Ran pnpm exec tsc --noEmit successfully.
• I did not run a full browser end-to-end test in this session.

Evidence

• CI passes (pnpm check && pnpm lint && npx tsc --noEmit)
• Manually tested locally
• Screenshots / recordings attached (if UI changes)

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have added/updated documentation as needed
• My changes do not introduce new warnings

[wyuc]

Thanks for the fix! This is covered by #114 which also threads providerType through all three call sites and refactors the chat route to use the shared resolveModel() helper. Closing as duplicate.