Add collapse and expand controls to markdown table of contents panel

[AmethystLiang]

Adds level-based collapse buttons (H1, H2, H3) and per-section disclosure chevrons to the TOC panel.

• markdown-toc-collapse-state.ts — pure functions for managing collapsed ID sets: toggle, prune stale IDs on items change, collapse to a given heading level.
• MarkdownTableOfContentsPanel.tsx — header row now has an H1/H2/H3 button group. Each parent node gets a ChevronRight button that toggles its children. Indentation adjusted for the chevron column.
• Collapse state resets when items change (prunes IDs no longer in the tree).
• Unit tests cover component rendering (aria labels, headings shown) and all collapse-state functions.

Testing: pnpm vitest run passes for the new test files.

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• Update PR summary snapshot

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[coderabbitai]

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Walkthrough

This pull request adds collapsible/expandable functionality to the Markdown table of contents. A new markdown-toc-collapse-state.ts module exports five utilities for managing collapse state: collecting parent node IDs, collapsing to a heading level, toggling items, pruning stale IDs, and determining expanded status. The MarkdownTableOfContentsPanel component now maintains local collapsedIds state, renders per-level collapse controls in the header, and passes handlers to child rows. TOC rows render disclosure chevrons and conditionally show children based on expanded status. Supporting unit tests verify both the utility functions and the component rendering. CSS updates provide styling for the new control buttons and adjusted row/disclosure button appearance.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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