NES: more detailed speculation prediction for patch-based prompts

[ulugbekna]

Adds an experiment-based knob to vary the "predicted output" speculation we send to the patch-based xtab model, so we can measure whether a richer hint improves first-token latency or accuracy.

What changed

• New PatchModelPrediction enum + experiment config chat.advanced.inlineEdits.xtabProvider.patchModelPredictionKind with four variants:
  • FilePath (today's behavior, default) — path/to/file:
  • CurrentLine — path:N\n-<line>
  • CurrentLineReplaced — path:N\n-<line>\n+
  • CurrentLineCompleted — path:N\n-<line>\n+<line>
• getPredictionContents plumbs cursorLineOffset (0-based, matching Patch.lineNumZeroBased parsed by XtabCustomDiffPatchResponseHandler) and cursorLineInEditWindowOffset through to the CustomDiffPatch branch.
• Guards against out-of-range cursorLineInEditWindowOffset so we never emit a literal -undefined into the prompt — falls back to the FilePath shape in that case.
• Renames cursorOriginalLinesOffset → cursorLineInEditWindowOffset across xtabProvider.ts to match what the value actually represents.
• Tests for all four CustomDiffPatch variants plus the fallback path.

fixes https://github.com/microsoft/vscode-internalbacklog/issues/7871

[Copilot]

Pull request overview

This PR adds an experiment-controlled switch to vary the “predicted output” hint sent to the Xtab patch-based (CustomDiffPatch) prompting strategy, enabling measurement of whether richer speculative hints improve latency/quality. It introduces a new PatchModelPrediction enum/config, plumbs cursor/line offsets into getPredictionContents, and expands unit coverage for the new variants and fallback behavior.

Changes:

• Added PatchModelPrediction (with validator) to represent multiple prediction shapes for patch-based prompts.
• Registered a new experiment-based setting chat.advanced.inlineEdits.xtabProvider.patchModelPredictionKind to control the prediction shape.
• Updated Xtab provider prediction generation to include cursor line info for CustomDiffPatch, and added/updated tests to cover all variants and out-of-range fallback.

Show a summary per file

File	Description
extensions/copilot/src/platform/inlineEdits/common/dataTypes/xtabPromptOptions.ts	Adds PatchModelPrediction enum and validator used to select the patch prediction shape.
extensions/copilot/src/platform/configuration/common/configurationService.ts	Registers the new experiment-based config key for selecting patch prediction kind.
extensions/copilot/src/extension/xtab/test/node/xtabProvider.spec.ts	Updates getPredictionContents tests to cover all CustomDiffPatch variants and fallback behavior.
extensions/copilot/src/extension/xtab/node/xtabProvider.ts	Plumbs cursor offsets into prediction generation and implements richer CustomDiffPatch prediction strings with bounds guarding.

Copilot's findings

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