Avoid nested scrolling in the code session rail

[BunsDev]

Summary

• let the shared SurfaceRail own scrolling for the wide Code sessions rail
• preserve independent scrolling for the narrow list-first layout
• pin both paths in the existing code surface contract test

Verification

• pnpm typecheck
• pnpm lint
• pnpm test:app
• pnpm test:api
• pnpm check:tests-wired

Bead: cave-iixug

[Copilot]

Pull request overview

This PR updates the Code surface sessions rail so that, in the wide layout, scrolling is owned by the shared SurfaceRail container (avoiding nested scroll areas), while the narrow “list-first” drill-in layout keeps its own independent scrolling. It also extends the existing surface contract test to pin this behavior.

Changes:

• In CodeSessionRail, conditionally remove overflow-y-auto when the rail is hosted inside SurfaceRail (detected via the presence of onExpand).
• Preserve overflow-y-auto for the standalone narrow layout where CodeSessionRail must remain scrollable.
• Add assertions to the code surface contract test to lock in the open-state scrolling class contract.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/components/code-session-rail.tsx	Introduces a layout-sensitive open-state class so wide SurfaceRail usage avoids nested scrolling while narrow standalone usage remains scrollable.
src/components/code-surface-mode.test.ts	Adds contract assertions to ensure the rail’s open-state scrolling behavior remains stable across both layout paths.

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