Replace the separate syntax_highlight module with the streaming markdown
formatter for final_output rendering. This:
- Removes special buffered rendering logic for final_output
- Uses the same StreamingMarkdownFormatter used for agent responses
- Removes the spinner animation (content renders immediately)
- Deletes the now-unused syntax_highlight.rs module
- Updates test to use the streaming formatter
Benefits:
- Consistent rendering across all markdown output
- Less code to maintain (removed ~250 lines)
- Same syntax highlighting via syntect (already in streaming formatter)
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