fix: enrich notebook model index error diagnostics and fix sort comparator (#301207)

[bryanchen-d]

Fixes #301207

Problem

Unhandled error model index out of range 11 in NotebookTextModel._assertIndex provides no diagnostic context — no cell count, no edit type, no indication of which lookup key produced the invalid index — making it difficult to determine the root cause from telemetry alone.

Additionally, the sort comparator in _doApplyEdits returned -1 when b.end === undefined (DocumentMetadata edits), which incorrectly sorted them after cell-referencing edits instead of before them.

Changes

1. Enrich _assertIndex error message — include cellCount and an optional context string so telemetry reports show the cell count and which edit type / lookup key triggered the failure.

2. Pass context to _assertIndex in the handle-lookup branch — so handle-based lookups also report diagnostic details.

3. Fix sort comparator — change return -1 to return 1 for the b.end === undefined case so DocumentMetadata edits (which have no cell index) sort before cell-referencing edits, matching the existing a.end === undefined branch.

[Copilot]

Pull request overview

Fixes an unhandled “model index out of range” error in notebook edit application by adding more diagnostic context to index validation and correcting edit ordering during application in NotebookTextModel.

Changes:

• Adds optional diagnostic context to _assertIndex error messages (including current cell count).
• Validates index-based (non-Replace/non-Move) edits earlier during edit mapping, similar to handle-based validation.
• Adjusts the edit sorting comparator for end === undefined (DocumentMetadata) edits.