fix(chat): preserve #file reference when editing text before it

[Anupam Kumar (AnupamKumar-1)]

Fixes #333941

Editing text before an inline #file: reference (for example, pressing
Backspace immediately before it or retyping an earlier word) could delete
the reference entirely.

The existing recovery logic handled replacements that enclosed the reference,
but did not preserve the reference when unrelated edits changed its offset.
Such edits could therefore fall through to the removal path.

This adds a touchesReference check to distinguish edits that overlap the
reference from edits that occur outside it. For non-overlapping edits, the
reference range is recomputed from the net offset delta instead of removing
the reference. Edits that touch the reference continue to remove it.

Added regression tests covering deletion before the reference, replacement
before it, deletion touching its first character, and edits both before and
after the reference in a single change event.

Manually verified the behavior with a real #file: reference in a running
dev build.

How to test

1. Open Chat.
2. Type a sentence, e.g. Compare the ownership section of .
3. Type #, pick a file so #file:README.md is inserted inline, then continue typing with the current draft.
4. Move the caret before the reference and edit there — e.g. press Backspace immediately before it, or retype a word right in front of it.
5. Confirm the #file:README.md reference is preserved instead of being deleted.

Also covered by the added tests in chatVariables.test.ts (delete-before, replace-before, delete-touching-first-char, edits-before-and-after).

[Copilot]

🟡 Changes recommended

Enclosing replacements can incorrectly trigger a second edit that deletes unrelated text.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Preserves inline chat references when unrelated edits shift their offsets.

Changes:

• Detects whether edits overlap a reference.
• Recomputes ranges after non-overlapping edits.
• Adds regression tests for related editing scenarios.

File summaries

File	Description
chatDynamicVariables.ts	Adds overlap detection and range recovery.
chatVariables.test.ts	Adds reference-editing regression tests.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Balanced

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[Copilot]

🟢 Approval recommended

The range-tracking fix is focused and adequately covered by regression tests.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0 new
• Review effort level: Balanced