feat(plugins): collapse long DeepSeek code blocks

Proposal

Add reversible long-code folding with individual toggle and expand/collapse all. Never truncate the original code or hide the native copy control. Preserve user-expanded state during streaming updates and teardown fully. Reuse existing code feature design, preferably a site-independent primitive with an opt-in DeepSeek manifest. Two-site fixtures and live page evidence are required.

Contribution workflow

The contributor requests implementation where feasible. This issue records scope before code. Maintainer confirmation of new primitive contracts is still needed; no maintainer approval is implied by opening this issue. New PRs will be submitted only after applicable checks pass, with real-browser gaps resolved rather than labelled as complete. Related DeepSeek integration: #960 and #995.