fix(app): restore legacy session header controls

[wgu9]

Issue for this PR

Closes #30158
Related #29808
Related #30545

Type of change

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

What does this PR do?

• Fixes the session header layout gate to read the newLayoutDesigns accessor value instead of passing the accessor function object to <Show>.
• Restores the legacy session header fallback, including the visible terminal, review, and file tree controls when the redesigned layout is disabled.
• Adds a small regression test for the layout gate.

How did you verify your code works?

• bun test src/components/session/session-header.test.ts from packages/app
• bun typecheck from packages/app
• bunx oxlint packages/app/src/components/session/session-header.tsx packages/app/src/components/session/session-header-layout.ts packages/app/src/components/session/session-header.test.ts
• git diff --check
• pre-push bun turbo typecheck

Checklist

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