test(desktop): scope slash menu refresh observer

[liuxiaocs7]

Summary

Scope the slash-command menu refresh regression watcher to the open menu and the original listbox and Skills group nodes. Wait for each session's real invocable-skills projection request before checking that those nodes remain connected, avoiding unrelated overlay removals and fixed timing delays.

Fixes #3727

Verification

• Targeted stress run: 20/20 passed
• Confirmed the test fails with the fix(desktop): slash picker alternates between two popup sizes after typing /r #2667 regression restored
• Desktop E2E target and isolated reruns passed
• Desktop unit: 1458/1458 passed
• Runtime, Runtime Host, Storage, and Eval suites passed
• Lint, format, Desktop typecheck, and ASF header checks passed

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex implemented the test synchronization and observer scoping changes, added the E2E-only preload signal, and assisted with verification.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[Astro-Han]

Thanks for the thorough fix! We chose to merge #3737 for this issue — it achieves the same correction (narrowing the observer to close the over-broad window) with zero production surface (1 file, no preload changes), which is minimal for a test-only flake. Your approach with a deterministic waitForInvocableSkillsCall is causally stronger, but we preferred to avoid expanding the preload test seam for this flake. Appreciate the effort — closing as superseded by #3737.