fix(web): preserve session preview folding

[NightWatcher314]

Summary

• keep the session preview limit in effect when auto-expanding the selected session
• do not let every active session bypass the preview limit
• still keep selected sessions and pending-request sessions visible inside the folded preview

Why #629 was not enough

This follows up on #629, which added the configurable “Sessions Before Folding” / session preview limit. That setting was applied in the basic folded-preview path, but two later/default visibility rules could effectively bypass it:

1. Selecting a session auto-expanded its machine/project path and also forced the per-directory session preview override to expanded, so the directory showed all sessions instead of the configured preview count.
2. The preview helper treated every active session as required-visible. In directories with many active sessions, the preview could grow far beyond the configured limit.

This PR keeps the useful auto-expansion of the containing machine/project, but preserves session preview folding. It also narrows the required-visible exception to pending-request sessions and the selected session.

Test plan

• bun test web/src/components/SessionList.test.ts
• bun run typecheck:web

[github-actions]

Findings

• No issues found.

Summary

• Review mode: initial
• Full diff reviewed: web/src/components/SessionList.tsx and web/src/components/SessionList.test.ts.
• Residual risk: validation is helper-level only; no rendered session-list interaction test covers selected-session visibility while preview folding remains collapsed.

Testing

• Not run (automation; PR content not executed during review).

HAPI Bot