fix: enable slash command chaining on any line in multiline input

[roomote-v0]

Related GitHub Issue

Closes: #12028

Description

This PR attempts to address Issue #12028. Feedback and guidance are welcome.

Previously, slash commands only worked on the first line of the textarea because the detection logic checked the entire text value (text.startsWith("/")) rather than the current line where the cursor sits.

Three targeted changes were made:

1. shouldShowContextMenu() (context-mentions.ts): Extracts the current line from the text based on cursor position and checks if that line starts with / (instead of checking the full text).

2. handleChange() (ChatTextArea.tsx): Same current-line extraction approach for slash command detection and search query building.

3. insertMention() (context-mentions.ts): When isSlashCommand is true, replaces only the slash token on the current line (from line start to cursor) instead of replacing the entire text content. This preserves content on other lines.

Test Procedure

• Updated existing tests in context-mentions.spec.ts to reflect the new per-line behavior
• Added new multiline tests for:
  • Slash command detection on second/third lines via shouldShowContextMenu
  • Slash command insertion preserving other lines via insertMention
  • Edge cases (slash with space on non-first line, non-slash text on second line)
• All 54 tests pass: cd webview-ui && npx vitest run src/utils/__tests__/context-mentions.spec.ts

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes.
• Documentation Impact: No documentation updates are required.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

The backend slash command parsing (src/core/mentions/index.ts) already uses commandRegexGlobal which handles multiline text correctly, so no backend changes were needed.

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