fix: Fix Enter key blocked after dismissing empty suggestion popup

[charlesvien]

Problem

Pressing Escape on a suggestion popup with no results hides it visually but leaves the Tippy DOM node mounted, causing the Enter-to-submit handler to think a suggestion is still active.

Changes

1. Track dismissed flag in CommandMention and FileMention suggestion renderers
2. Return false from onKeyDown when dismissed so Enter propagates normally
3. Return false from SuggestionList when no item is selected instead of swallowing the event
4. Check for visible Tippy box (data-state not hidden) instead of just DOM presence
5. Reset dismissed flag when new non-empty suggestions arrive

How did you test this?

Manually

[charlesvien]

• fix: Show parent directory in file mention chips #1472
• fix: Fix Enter key blocked after dismissing empty suggestion popup #1471 👈 (View in Graphite)
• fix: Open active file in external app instead of repo directory #1470
• fix: Rename bypass permissions to auto-accept permissions #1469
• fix: Polish message editor, settings dialog and minor UX tweaks #1468
• main

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[charlesvien]

Merge activity

• Apr 3, 1:05 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Apr 3, 1:19 AM UTC: Graphite rebased this pull request as part of a merge.
• Apr 3, 1:25 AM UTC: @charlesvien merged this pull request with Graphite.