nes-datagen: capture cross-file next edits (xtab-cross-file sample task)

[g1910]

Summary

Extends the nes-datagen simulation pipeline to produce cross-file next-edit
training samples via a new xtab-cross-file sample task. Until now the pipeline only
labeled the next edit inside the active file; this captures edits that span multiple files.

For each recording the pipeline now:

• Buckets post-request edits per document and composes per-file replacements.
• Resolves each target file's request-time content.
• Emits a multi-file CustomDiffPatch response (one filename:line patch block per touched file).
• Records targetFiles / targetFilePaths / isCrossFile in the sample metadata,
  ordered to match the response blocks.

A new --patch-order option (first-touch (default) | anchor-first) controls per-file
block ordering.

What changed

Pipeline:

• test/base/simulationOptions.ts — xtab-cross-file task + --patch-order option, with validation and help text.
• test/pipeline/alternativeAction/processor.ts, types.ts — bucket multi-file edits into per-file fileEdits[].
• test/pipeline/replayRecording.ts — expose each target's request-time content (targetFileEdits).
• test/pipeline/responseStep.ts — multi-file CustomDiffPatch generation (anchor-only fallback when not cross-file).
• test/pipeline/pipeline.ts — task dispatch + orderTargetFiles (first-touch / anchor-first).
• test/pipeline/output.ts — discriminated-union SampleClassification arm carrying cross-file metadata.

Tests:

• test/pipeline/test/xtabCrossFilePipeline.e2e.spec.ts + test/pipeline/test/fixtures/xtabCrossFileFixtureData.ts
  — synthetic, oracle-based e2e covering the multi-file label, --patch-order ordering,
  cross-file metadata, and the xtab single-file fallback. No model/network calls.

How to test

From extensions/copilot/:

npx vitest run test/pipeline --pool=forks

All pipeline suites pass (87 passed, 1 skipped), including the new xtab-cross-file cases.

Generate cross-file samples from a recording:

npm run simulate -- --config-file=config.json nes-datagen --input=data.jsonl --sample-task=xtab-cross-file --patch-order=anchor-first

(--config-file must include ...xtabProvider.modelConfiguration; output defaults to <input>_output.jsonl.)

Notes

• Backward compatible: default sample task remains xtab; existing xtab / cursor-* tasks are unchanged.
• No recordings/telemetry or model config are included in this PR.

[Copilot]

Pull request overview

This pull request extends the nes-datagen training-data simulation pipeline in extensions/copilot/test/pipeline/ to support a new xtab-cross-file sample task that can label the “next edit” across multiple files, including ordering control via --patch-order, and emits corresponding per-file metadata.

Changes:

• Adds xtab-cross-file task selection + --patch-order option to simulation CLI options.
• Buckets post-request edits across files, reconstructs request-time content per file, and generates multi-file CustomDiffPatch assistant labels.
• Adds an end-to-end test and synthetic fixture data to validate multi-file labeling, ordering policies, and the single-file xtab fallback.

Show a summary per file

File	Description
extensions/copilot/test/base/simulationOptions.ts	Adds xtab-cross-file task and --patch-order option parsing/help text.
extensions/copilot/test/pipeline/alternativeAction/types.ts	Extends NextUserEdit to include per-file composed edits (fileEdits).
extensions/copilot/test/pipeline/alternativeAction/processor.ts	Buckets post-request edits by file, composes per-file replacements, preserves first-touch order.
extensions/copilot/test/pipeline/replayRecording.ts	Captures targetFileEdits with request-time content per touched file for label generation.
extensions/copilot/test/pipeline/responseStep.ts	Introduces multi-file response generation for CustomDiffPatch labels.
extensions/copilot/test/pipeline/pipeline.ts	Dispatches xtab-cross-file, orders target files (first-touch/anchor-first), wires response + metadata classification.
extensions/copilot/test/pipeline/output.ts	Adds discriminated-union classification payload for cross-file metadata.
extensions/copilot/test/pipeline/test/xtabCrossFilePipeline.e2e.spec.ts	New e2e test validating multi-file label ordering + metadata + xtab fallback.
extensions/copilot/test/pipeline/test/fixtures/xtabCrossFileFixtureData.ts	Synthetic fixture recordings for cross-file and anchor-only scenarios.

Review details

• Files reviewed: 9/9 changed files
• Comments generated: 2
• Review effort level: Low