fix(ui): collapse thinking blocks by default for reasoning models

[VedantMadane]

Fixes #3977

Removes the \open\ attribute from the \ThinkBlock\ <details>\ element so that <think>\ content from reasoning models (DeepSeek R1, etc.) is collapsed by default. Users can still expand to view the full thinking process by clicking the summary.

Change

One-line change in ^[e/tabby-ui/components/message-markdown/index.tsx:

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The existing <think>\ tag pipeline (CUSTOM_HTML_BLOCK_TAGS, formatCustomHTMLBlockTags, rehypeSanitize, ThinkBlock component) already handles parsing correctly. The only issue was the default visibility.

[mellon-builds]

Collapsing <think> blocks by default is the right UX call — the raw chain-of-thought from reasoning models is useful for debugging but distracting as the default view. The fix is minimal and correct. One follow-up worth considering: should the collapse state persist across page reloads, or reset to collapsed each time? Persisting the user's preference per conversation could be useful for workflows where reviewers want to see the reasoning consistently.

[VedantMadane]

Thanks for the feedback. I agree that persisting the preference would be a great UX addition. However, to keep this PR atomic and minimal, I propose keeping it as a default-collapsed state for now. We can definitely explore persistence in a follow-up PR.