fix: prevent autocomplete from highlighting items without mouse movement

[roomote]

Description

This PR attempts to address Issue #8615. Feedback and guidance are welcome.

When the autocomplete menu opens (for @ mentions or / commands), items under the mouse cursor would be automatically highlighted even without any mouse movement. This caused the wrong item to be selected when pressing Enter, as reported in the issue.

Problem

• Mouse cursor position was causing immediate item highlighting on menu open
• No distinction between intentional hover (mouse movement) and accidental positioning
• Different behavior from VS Code's standard autocomplete (e.g., Ctrl+P)

Solution

Added mouse movement tracking to the ContextMenu component:

• New hasMouseMoved state tracks whether the mouse has actually moved
• Items only highlight on hover after mouse movement is detected
• State resets when menu changes (new search query)

Testing

1. Type @ or / in the chat input
2. Position mouse over where autocomplete items will appear before typing
3. Verify that no item is highlighted until you move the mouse
4. Move mouse slightly and verify hover highlighting now works normally

Checklist

• Code follows project conventions
• All tests pass (1094 tests passed)
• Linting passes
• Type checking passes

Fixes #8615

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Important

Fixes autocomplete highlighting issue by tracking mouse movement in ContextMenu.tsx.

• Behavior:
  • Fixes issue where autocomplete items were highlighted without mouse movement in ContextMenu.tsx.
  • Introduces hasMouseMoved state to track mouse movement.
  • Items only highlight after mouse movement is detected.
  • Resets hasMouseMoved state when menu changes (new search query).
• Functions:
  • Adds handleMouseMove() to update hasMouseMoved state.
  • Modifies onMouseEnter to check hasMouseMoved before highlighting items.

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