Add <final> tag handling to stripThinkingTags() to prevent reasoning-tag
provider responses from leaking incomplete tags during streaming.
When using providers like google-antigravity/*, ollama, or minimax, the
model wraps responses in <think>...</think> and <final>...</final> tags.
The TUI was only stripping <think> tags, causing <final> to leak through
and display as the response ~50% of the time.
This is a defense-in-depth fix for the TUI layer.
Fixes: #1561
Co-authored-by: Claude Code <noreply@anthropic.com>
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