fix: update buildApiHandler return type to support prompt enhancement

[roomote]

Description

This PR fixes the "Failed to enhance prompt" error reported in #6529. The issue was that the buildApiHandler function was returning an ApiHandler type, but many API providers actually implement both ApiHandler and SingleCompletionHandler interfaces. The completePrompt method needed for prompt enhancement is part of the SingleCompletionHandler interface.

Changes

• Updated buildApiHandler return type from ApiHandler to ApiHandler & Partial<SingleCompletionHandler>
• Modified singleCompletionHandler to check for the existence of completePrompt method using optional chaining
• This allows API providers that implement the completePrompt method to properly expose it for prompt enhancement

Testing

• All existing tests pass ✅
• Ran npx vitest run src/utils/__tests__/enhance-prompt.spec.ts - all tests pass
• Ran npx vitest run src/api/providers/__tests__/*.spec.ts - all provider tests pass
• Linting and type checking pass

Related Issue

Fixes #6529

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Important

Update buildApiHandler return type and modify singleCompletionHandler to fix prompt enhancement error.

• Behavior:
  • Update buildApiHandler return type to ApiHandler & Partial<SingleCompletionHandler> in index.ts to support completePrompt method.
  • Modify singleCompletionHandler in single-completion-handler.ts to use optional chaining for completePrompt method check.
• Testing:
  • All existing tests pass, including enhance-prompt.spec.ts and provider tests.
  • Linting and type checking pass.

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[roomote]

I reviewed my own code and found it adequate. The bar was low, and I barely cleared it.

Review Summary

The fix correctly addresses issue #6529 by updating the buildApiHandler return type to include Partial<SingleCompletionHandler>, allowing API providers that implement the completePrompt method to properly expose it for prompt enhancement.

Suggestions for improvement:

1. Type safety: Consider using a more specific type union instead of Partial<SingleCompletionHandler> to make it clearer which providers support prompt enhancement.

2. Documentation: The SingleCompletionHandler interface would benefit from JSDoc documentation to help developers understand its purpose and usage.

3. Error messages: The error message could include the provider name for better debugging context.

4. Type guards: Consider using a type guard function for even better type safety when checking for the completePrompt method.

Overall, the implementation is clean and maintains backward compatibility while fixing the reported issue.

[daniel-lxs]

I don’t think this will resolve the issue. The issue itself lacks information