fix: avoid redundant table cell content renders

[AkashNemade16]

Fixes: #8890

Opening a dialog from a cell reruns render functions in otherwise unchanged cells, rebuilding nested content such as tabs. Memoize the cell render values and row ButtonContext, and keep the empty expand-button props stable for non-tree rows.

This skips unchanged cell content and button updates, not table state updates. Focus and selection behavior are unchanged; Cell hooks and DOM rendering still run.

With three stable rows, opening a per-cell/shared dialog drops render-prop calls across six unrelated cells from 18/36 to zero in StrictMode. The regressions cover both setups, repeated mouse/keyboard opening and closing, and focus restoration.

GitHub Copilot assisted with implementation and validation.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests). Tests added; the existing TableExample covers the unchanged UI.
• Filled out test instructions.
• Updated documentation (if it already exists for this component). N/A: no API or usage change.
• Looked at the Accessibility Practices for this feature - Aria Practices
• I understand every change in this PR and can explain why it's there.
• If AI-assisted, I followed our AI contribution guidance and pointed my assistant at AGENTS.md.

📝 Test Instructions:

Recorded on fccdedafe with Node 24.14.1, before the subsequent main merge:

• yarn test --maxWorkers=2 --verbose=false: 8,288 tests and 262 snapshots passed; 16 tests skipped. Non-fatal worker teardown warning.
• yarn test:ssr --verbose=false: all 60 suites and 74 tests passed.
• yarn lint: passed.

Full browser validation remains incomplete. Firefox fails to launch on this macOS host with a sandbox-extension permission error, also reproduced in standalone Playwright before loading repository code. A later Chromium/WebKit run reported four WebKit Chat scroll/loading failures and stalled; these have not been established as pre-existing failures. The 14 TableView, Modal, Dialog, and Tabs cases passed serially on Chromium and WebKit.

For manual review, use TableExample: open a cell's Delete dialog, close with Escape, and reopen with Enter. Check focus returns to the trigger and arrow navigation still works.