Add telemetry to diagnose intermittent NES model switching

[ulugbekna]

Problem

Investigation of provideinlineedit telemetry revealed that ~0.04% of sessions with successful NES fetches experience intermittent model switching — the NES model bounces between two different models (e.g., nes-callisto-003 → nes-callisto → nes-callisto-003) mid-session.

We couldn't determine the root cause from existing telemetry because the modelConfig property didn't include how the model was selected (fetched from /models, experiment config, hardcoded default, etc.).

Changes

1. nesModelChanged telemetry event

Fires whenever _currentModelObs produces a different model than the previous one. Captures:

• previousModel / newModel — model names before and after
• newModelSource — how the model was picked (fetched, expConfig, expDefaultConfig, localConfig, hardCodedDefault)
• modelListNames / modelListSources — full snapshot of the aggregated model list
• fetchedNesModels — raw NES models from latest /models response
• hasCopilotToken / isFreeUser — token state at switch time

2. source field in modelConfig JSON on every provideInlineEdit event

The existing modelConfig telemetry property now includes a source field, so every NES request self-documents how its model was chosen. Query via:

| extend modelSource = tostring(parse_json(Properties["modelconfig"]).source)

3. selectedModel() API on IInlineEditsModelService

Replaces the old selectedModelConfiguration() with selectedModel() returning { config, source } atomically from a single observable read — avoids a race between config and source.

Possible causes under investigation

1. Server /models endpoint returning different NES models across token refreshes
2. Transient loss of /models data falling through to hardcoded defaults
3. Experiment assignment changes mid-session

This telemetry will confirm which.

[Copilot]

Pull request overview

Adds diagnostic telemetry around Next Edit Suggestions (NES) model selection to help identify the cause of rare mid-session model switching, and updates the inline edits model service API to return model + source atomically.

Changes:

• Add a new nesModelChanged MSFT telemetry event emitted when the selected NES model name changes.
• Include a source field in the serialized modelConfig telemetry for each provideInlineEdit flow.
• Replace selectedModelConfiguration() with selectedModel() returning { config, source } to avoid config/source races.

Show a summary per file

File	Description
extensions/copilot/src/platform/inlineEdits/node/inlineEditsModelService.ts	Adds nesModelChanged telemetry via an autorun and introduces the new selectedModel() implementation.
extensions/copilot/src/platform/inlineEdits/common/inlineEditsModelService.ts	Updates the service interface to selectedModel(): SelectedModel.
extensions/copilot/src/extension/xtab/node/xtabProvider.ts	Plumbs model source into modelConfig telemetry JSON and switches to selectedModel().
extensions/copilot/src/extension/xtab/test/node/xtabProvider.spec.ts	Updates mocks and tests to use selectedModel() instead of selectedModelConfiguration().

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2