agentHost: preserve complete multi-root changes summary

[Don Jayamanne (DonJayamanne)]

Summary

• publish the multi-root all-folder changes summary only when every source succeeds
• preserve the last complete live and persisted summary when any repository diff is unavailable
• cover cold-cache and warm-cache partial-failure behavior

Testing

• npm run compile-client
• npm run hygiene
• VSCODE_SKIP_PRELAUNCH=1 ./scripts/test.sh --run src/vs/platform/agentHost/test/node/agentHostChangesetService.test.ts --grep "all-folder branch summary" (12 passing)

Fixes #332694

[Copilot]

Copilot review overview

Review tier: Balanced
Findings: None

What changed in this PR

Ensures multi-root Agent Host summaries remain complete when repository diffs fail.

Changes:

• Publishes summaries only when every source succeeds.
• Preserves cached live and persisted summaries on partial failure.
• Adds cold- and warm-cache failure coverage.

File	Description
agentHostChangesetService.ts	Requires complete multi-root diff results.
agentHostChangesetService.test.ts	Tests partial-failure preservation behavior.

Suppressed comments (1)

src/vs/platform/agentHost/node/agentHostChangesetService.ts:995

• This branch now also handles partial, but the warning still claims that no source was available. That makes partial failures indistinguishable from total failures in diagnostics—the exact scenario this change addresses. Report the actual evaluation outcome and update the comment accordingly.

		if (evaluation.outcome !== 'complete') {
			// No source produced diffs (total failure or no sources at all).
			// Preserve the previously cached summary instead of clobbering it
			// with a spurious zero aggregate.
			this._logService.warn(`[AgentHostChangesetService] No diff source available for multi-folder branch summary ${session}; preserving the cached summary.`);

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