Fix insertSnippet() destroying active snippet session tabstops

[almayor]

Problem

Related: #322010

Since VS Code 1.86, calling editor.insertSnippet() from an extension while a snippet session is active destroys the outer session's remaining tabstops. This is a regression — it worked correctly in 1.85.2 and earlier.

This breaks extensions like HyperSnips, LTeX, and others that programmatically insert snippets, making nested snippet expansion unusable.

Root cause

In 1.86, mainThreadEditor.ts was refactored to route insertSnippet() calls through snippetController.apply(edits[]) instead of snippetController.insert(template). The apply() path calls this.cancel() when a session is active:

if (this._session && typeof template !== 'string') {
    this.cancel();
}

The insert() path instead calls this._session.merge(template, opts), which properly nests the new snippet inside the active placeholder. The merge infrastructure (OneSnippet.merge(), _nestingLevel, _renormalizePlaceholderIndices()) is fully implemented and works — the regression was the API bridge switching code paths.

Fix

Route single-edit insertSnippet() calls back through snippetController.insert() (the merge path), falling back to apply() only for multi-range edits.

Note on issue #214757

This bug was reported in #214757, which was closed as a duplicate of #63129. However, #63129 is about snippet auto-indentation (fixed in #234858), not tabstop destruction — they are unrelated issues. The bug remains present through current versions.

Fixes #214757

Test

1. Install HyperSnips or any extension that calls editor.insertSnippet() during editing
2. Trigger a snippet with multiple tabstops (e.g. \int_{$1}^{$2} $3 d$4)
3. While cursor is at $1, trigger another snippet insertion via the extension
4. Press Tab — cursor should advance to $2 (currently it doesn't; the session is destroyed)

---

This fix was researched and developed with the assistance of Claude Code (Claude Opus 4.6).

[Copilot]

Pull request overview

This PR addresses a regression (since 1.86) where extension calls to TextEditor.insertSnippet() during an active snippet session can cancel the outer session and destroy remaining tabstops. The approach is to route single-range snippet insertions through the merge-capable SnippetController2.insert(...) path instead of the edits-array apply(...) path that cancels active sessions.

Changes:

• Refactors MainThreadTextEditor.insertSnippet to build a shared insertOptions object for snippet insertion.
• Routes single-range insertions through snippetController.insert(template, ...) after setting the editor selection to the target range.
• Keeps multi-range insertions on the snippetController.apply(edits, ...) path.

[almayor]

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