fix: strip thinking blocks from qwen-code completePrompt responses

[roomote-v0]

Related GitHub Issue

Closes: #12102

Description

This PR attempts to address Issue #12102 where prompt enhancement fails on qwen3-coder models with a "Failed to enhance prompt" toast.

Root cause: The QwenCodeHandler.completePrompt() method returns raw response content without any post-processing. qwen3-coder models are thinking/reasoning models that include <think>...</think> blocks in their responses. This causes either:

1. The content field to be mostly thinking markup, producing garbled enhanced prompts
2. The actual useful text to be buried inside thinking blocks

Fix: Added a regex strip of <think>...</think> blocks from the completePrompt() response, consistent with how createMessage() already handles thinking blocks during streaming (lines 257-283 in qwen-code.ts).

The approach mirrors how other providers handle thinking models in their completePrompt methods (e.g., Mistral filters out thinking content).

Feedback and guidance are welcome.

Test Procedure

• Added new test file src/api/providers/__tests__/qwen-code-complete-prompt.spec.ts with 6 tests covering:
  • Plain text responses returned as-is
  • Single <think> block stripped from response
  • Multiple <think> blocks stripped
  • Multiline <think> blocks handled
  • Response that is only a think block returns empty string
  • Null content returns empty string
• All existing qwen-code tests continue to pass
• Run tests: cd src && pnpm exec vitest run api/providers/__tests__/qwen-code-complete-prompt.spec.ts

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• 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.
• Documentation Impact: No documentation updates are required.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

This is a minimal, targeted fix. The change is a single line addition to completePrompt() that strips thinking blocks via regex before returning the response text.

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