nes: emitFastCursorChange: support additive only emitting

[ulugbekna]

Improved isAdditiveEdit to use subsequence matching instead of simple substring containment. This correctly detects additive edits where text is inserted in the middle of a line.

Example: 'function fib() {' → 'function fib(n: number) {' is now correctly identified as additive.

The algorithm is O(n) with O(1) space - a single pass through the new line checking if all original characters appear in order.

[Copilot]

Pull request overview

This PR extends “fast cursor line change” emitting to support an additive-only mode by improving additive-edit detection from substring containment to subsequence matching (so insertions in the middle of a line qualify).

Changes:

• Replaces the emitFastCursorLineChange boolean with an EmitFastCursorLineChange enum (off / always / additive-only) and adds a boolean→enum mapping helper.
• Adds isAdditiveEdit using subsequence matching and gates fast-cursor emission when configured to additive-only.
• Adds unit tests covering isAdditiveEdit behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/platform/inlineEdits/common/responseProcessor.ts	Introduces enum-based fast-cursor emitting and subsequence-based additive detection.
src/platform/configuration/common/configurationService.ts	Updates the xtab config key type/default for fast-cursor emitting.
src/extension/xtab/node/xtabProvider.ts	Maps the config value into the new enum-based diff options.
src/extension/xtab/test/common/responseProcessor.spec.ts	Adds test coverage for additive edit detection.

Comments suppressed due to low confidence (1)

src/platform/configuration/common/configurationService.ts:798

• This setting now accepts a string enum at runtime, but no validator is provided. A typo or unexpected value (including legacy boolean values from existing user settings) will flow through unvalidated and can result in unintended behavior. Consider adding a validator (e.g., vUnion(vBoolean(), vEnum('off','always','additive-only'))) and/or changing the config type to boolean | ResponseProcessor.EmitFastCursorLineChange so legacy boolean values are explicitly supported and invalid strings fall back to the default.

		export const InlineEditsXtabProviderEmitFastCursorLineChange = defineTeamInternalSetting<ResponseProcessor.EmitFastCursorLineChange>('chat.advanced.inlineEdits.xtabProvider.emitFastCursorLineChange', ConfigType.ExperimentBased, ResponseProcessor.DEFAULT_DIFF_PARAMS.emitFastCursorLineChange);
		export const InlineEditsXtabIncludeViewedFiles = defineTeamInternalSetting<boolean>('chat.advanced.inlineEdits.xtabProvider.includeViewedFiles', ConfigType.ExperimentBased, xtabPromptOptions.DEFAULT_OPTIONS.recentlyViewedDocuments.includeViewedFiles);