fix(opencode): handle CJK prefixes in TUI @ autocomplete

[yaohwu]

Issue for this PR

Closes #26808

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

• fix TUI @ autocomplete when the prompt contains CJK text before the trigger
• use display-width-safe slicing in the autocomplete query and guard checks
• add a regression test covering CJK-prefixed mention and slash parsing

How did you verify your code works?

• Reproduced in the TUI before the fix with input like 你好 @rea, which showed No matching items
• Confirmed the same flows now work with:
  • abc @rea
  • 你好 @rea
  • 你好@rea
  • 你好 abc @pack
• Ran:
  • bun test test/cli/run/prompt.shared.test.ts
  • bun test test/cli/run test/cli/cmd/tui/prompt-traits.test.ts test/cli/cmd/tui/prompt-part.test.ts test/cli/tui/prompt-submit-race.test.ts
  • bun typecheck

Screenshots / recordings

before：

after fixed:

Checklist

• [ x] I have tested my changes locally
• [ x] I have not included unrelated changes in this PR

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• No issue referenced. Please add Closes #<number> linking to the relevant issue.
• Not all checklist items are checked. Please confirm you have tested locally and have not included unrelated changes.

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found one potentially related PR worth noting:

PR #27649: Fix multiline mentions

• Fix multiline mentions #27649
• This PR addresses mention-related functionality in the TUI, which is closely related to the @ autocomplete handling in the current PR fix(opencode): handle CJK prefixes in TUI @ autocomplete #27651

However, PR #27649 focuses on multiline mentions while PR #27651 specifically addresses CJK prefix handling in @ autocomplete, so they appear to be addressing different aspects of the mention feature rather than being duplicates.

All other search results returned only the current PR #27651 itself, indicating no other significant duplicates exist for this specific CJK prefix autocomplete fix.