fix: sanitize tool tags from XAI reasoning content

[roomote]

Description

This PR addresses Issue #9041 where the thinking block for Grok Code Fast 1 was displaying tool-related XML tags like <appy_diff> and <switch_mode> instead of just the model's reasoning content.

Problem

When using Grok Code Fast 1, the reasoning/thinking collapsible blocks sometimes showed tool command tags mixed with actual thinking content, making it difficult to read the model's actual reasoning process.

Solution

• Added a sanitizeReasoningContent() function in the XAI provider that removes known tool-related XML/HTML tags while preserving the text content between them
• The function uses a targeted regex pattern to match and remove specific tool tags (apply_diff, switch_mode, write_to_file, etc.)
• Integrated sanitization into the streaming pipeline before yielding reasoning content to the UI
• Added logic to skip empty reasoning chunks after sanitization

Testing

• Added comprehensive unit tests covering:
  • Simple tag removal with preservation of inner text
  • Complex nested/multiline tool tag blocks
  • Empty-after-sanitization scenarios
  • Preservation of clean reasoning content
• All existing tests continue to pass

Result

The thinking blocks now accurately display only the model's reasoning without extraneous tool command tags, as requested in the issue.

Fixes #9041

---

Important

Adds sanitizeReasoningContent() to remove tool-related tags from reasoning content in XAIHandler, with tests for various scenarios.

• Behavior:
  • Adds sanitizeReasoningContent() in xai.ts to remove tool-related XML/HTML tags from reasoning content.
  • Integrates sanitization in createMessage() in XAIHandler to clean reasoning content before yielding.
  • Skips yielding reasoning if content is empty after sanitization.
• Testing:
  • Adds unit tests in xai.spec.ts for tag removal, nested tags, empty content, and clean content preservation.
  • Ensures all existing tests pass.

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[roomote]

Review complete. No issues found.

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[ellipsis-dev]

Typographical error: It appears that appy_diff is used instead of apply_diff. Please correct this to maintain consistency with other tests.

Suggested change

	const onlyTags = "<appy_diff></appy_diff><switch_mode></switch_mode>"
	const onlyTags = "<apply_diff></apply_diff><switch_mode></switch_mode>"

[daniel-lxs]

#9041 (comment)