perf(app): navigation between sessions + fix terminal test regressions

[neriousy]

Issue for this PR

Closes #16924

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Navigation between sessions is really slow.
The major bottleneck is rendering the parts

Also fixes terminal tests regressions

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How did you verify your code works?

Switch between sessions

Screenshots / recordings

2026-03-10.20-11-27.mp4

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

[github-actions]

Hey! Your PR title perf(app): navigation between sessions doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[Copilot]

Pull request overview

Improves session navigation performance by deferring and reducing expensive message-part rendering work in the UI.

Changes:

• Incrementally renders grouped assistant parts using a capped “visible” slice advanced via requestAnimationFrame.
• Lazily mounts Part content using IntersectionObserver with a timeout fallback and placeholders.
• Reduces repeated Array.includes checks by memoizing a Set for expanded file paths.

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