Fix code search for case where we can't get a local diff

[mjbvz]

For #313281

[Copilot]

Pull request overview

This PR updates Copilot's workspace chunk search fallback path so code search can continue when the workspace diff tracker cannot produce a local diff, which is relevant to the external-ingest work tracked in #313281 for non-standard/virtual file systems.

Changes:

• Introduces a LocalDiffResult type so local-diff lookup can explicitly return unknown or tooLarge.
• Changes workspace search to keep running when no concrete diff file list is available instead of bailing out immediately.
• Adjusts external-ingest/local-result filtering so merge logic can operate with an optional diff pattern.

Show a summary per file

File	Description
extensions/copilot/src/platform/workspaceChunkSearch/node/codeSearch/codeSearchChunkSearch.ts	Extends local-diff handling and fallback merge behavior in Copilot workspace/code search.

Copilot's findings

Comments suppressed due to low confidence (2)

extensions/copilot/src/platform/workspaceChunkSearch/node/codeSearch/codeSearchChunkSearch.ts:715

• This fallback only helps after searchWorkspace() has already started, but searchWorkspace() still calls isAvailable() first and isAvailable() still rejects 'unknown'/'tooLarge' diffs unless external ingest is enabled. Because external ingest is off by default, the original 'can't get a local diff' failure still reproduces for the normal code-search-only configuration.

		if (Array.isArray(diffArray)) {
			// Force it to search the local diff too so we can override stale code-search results.
			await raceCancellationError(this._externalIngestIndex.value.updateForceIncludeFiles(diffArray, token), token);
		}

		const externalResult = await this._externalIngestIndex.value.search(sizing, query, innerTelemetryInfo, token);
		if (externalResult) {
			if (!Array.isArray(diffArray)) {
				return { chunks: externalResult, strategyId: 'externalIngest' };

extensions/copilot/src/platform/workspaceChunkSearch/node/codeSearch/codeSearchChunkSearch.ts:710

• Skipping updateForceIncludeFiles() for non-array diff states leaves whatever files were force-included by the previous successful diff in ExternalIngestIndex. Since this new path later accepts all external-ingest hits when diffFilePattern is undefined, an old diff snapshot can keep overriding current remote results after the local diff becomes unavailable.

		if (Array.isArray(diffArray)) {
			// Force it to search the local diff too so we can override stale code-search results.
			await raceCancellationError(this._externalIngestIndex.value.updateForceIncludeFiles(diffArray, token), token);
		}

• Files reviewed: 1/1 changed files
• Comments generated: 1