Fix displaying diff line when instant applying in jetbrains extensions

[Pyroboomka]

Before:

After:

Should also resolve same issue for multi-edit tool, since they use the same path. Should not affect edit_existing_file in any way.
No the cleanest solution, but it works.

Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Tests

no testing, trust me (c)

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Summary by cubic

Fixes missing added-line highlights when instant-applying diffs in the JetBrains extension. Also fixes the same issue in the multi-edit tool; no impact to edit_existing_file.

• Bug Fixes
  • Send ADD lines to addNewLineForHighlightOnly with the current line to render the highlight during streaming.
  • Add addNewLineForHighlightOnly in VerticalDiffBlock to apply the green CONTINUE_DIFF_NEW_LINE highlighter and record the text.

^{Written for commit cc0b12e. Summary will update on new commits.}

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[continue-staging]

Documentation Review

No documentation updates needed.

This PR is an internal bug fix for the JetBrains extension's diff highlighting behavior during instant apply and multi-edit operations. The changes are confined to internal Kotlin implementation code (DiffStreamHandler.kt and VerticalDiffBlock.kt) and correct a visual rendering issue where added lines weren't being properly highlighted.

Since this fix:

• Doesn't change any user-facing features or workflows
• Doesn't modify configuration options or APIs
• Simply corrects existing behavior to work as expected

The Continue Docs remain accurate as-is.

[continue-staging]

Documentation Review: No docs updates needed.

This PR fixes an internal visual rendering bug in the JetBrains extension where added lines weren't being properly highlighted during instant-apply diff streaming. The changes are:

• Internal Kotlin implementation details (adding a helper method for line highlighting)
• No user-facing API, configuration, or workflow changes
• No new features that users need to learn about

The existing documentation accurately describes the diff/instant-apply functionality—this fix simply makes the visual feedback work correctly as already documented.

[continue-staging]

Documentation Review

No documentation update needed.

This PR is a visual bug fix for the JetBrains extension's diff line highlighting during instant apply. The changes:

• Fix missing green highlighting on added lines when streaming diffs
• Are purely internal rendering improvements
• Don't introduce new features, APIs, or configuration options
• Don't change any user-facing behavior that's currently documented

The existing documentation accurately describes the edit/diff functionality—this PR simply fixes how it's visually displayed in the editor.

[cubic-dev-ai]

No issues found across 2 files

[RomneyDa]

@Pyroboomka seems like a fine approach, thanks for the fix!