feat: support additionalContext in PreCompact hook

[roblourens]

Migrated from microsoft/vscode-copilot-chat#4980
Original author: @abwuge

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Allow PreCompact hook to inject additional context into the summarization prompt, matching the pattern used by other hooks (PreToolUse, PostToolUse, SessionStart, SubagentStart, UserPromptSubmit).

Changes:

• Add IPreCompactHookSpecificCommandOutput type in hookCommandTypes.ts
• Add PreCompactHookOutput interface in chatHookService.ts
• Modify executePreCompactHook() to collect additionalContext from hook results
• Merge hook additionalContext into summarizationInstructions in summarizeHistory()
• Add unit tests for prompt rendering with summarizationInstructions
• Add integration tests for PreCompact hook flow in preCompactHook.spec.tsx

[Copilot]

Pull request overview

Adds support for PreCompact hooks to contribute additionalContext that is merged into the conversation compaction/summarization prompt, aligning PreCompact with other hook types that can inject prompt context.

Changes:

• Introduces PreCompact hook-specific output typing (additionalContext) for hook command results.
• Executes PreCompact hooks before summarization and merges returned additionalContext into summarizationInstructions.
• Adds unit and integration tests validating prompt rendering and PreCompact hook flow.

Show a summary per file

File	Description
extensions/copilot/src/platform/chat/common/hookCommandTypes.ts	Adds IPreCompactHookSpecificCommandOutput type to the hook command JSON contract types.
extensions/copilot/src/platform/chat/common/chatHookService.ts	Adds PreCompactHookOutput interface describing nested hookSpecificOutput.additionalContext.
extensions/copilot/src/extension/prompts/node/agent/summarizedConversationHistory.tsx	Runs PreCompact hooks before summarization and merges hook-provided context into summarization instructions; updates prompt heading text.
extensions/copilot/src/extension/prompts/node/agent/test/summarization.spec.tsx	Adds tests for rendering the “additional summarization instructions” section when provided/omitted.
extensions/copilot/src/extension/prompts/node/agent/test/preCompactHook.spec.tsx	Adds integration tests ensuring PreCompact is invoked and its additionalContext influences the summarization prompt.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2

[Copilot]

Avoid importing MockChatHookService from another *.spec.ts file. Importing a spec as a module can cause that spec’s tests to be registered/executed as a dependency (and then again when the runner loads it as a test file), which can lead to duplicated suites and flaky ordering. Move MockChatHookService into a dedicated test helper module (non-*.spec.*) and import it from both specs.

[Copilot]

executePreCompactHook manually iterates hook results and only logs resultKind === 'error'. This drops warning results (and any stopReason/warningMessage) on the floor, unlike other hook call sites (e.g. SessionStart/SubagentStart) that use processHookResults(..., ignoreErrors: true) to consistently handle warnings/errors while still proceeding. Consider switching to processHookResults here (with outputStream: undefined and ignoreErrors: true) and collecting hookSpecificOutput.additionalContext in onSuccess.

Suggested change

	if (result.resultKind === 'error') {
	const errorMessage = typeof result.output === 'string' ? result.output : 'Unknown error';
	this.logService.error(`[ConversationHistorySummarizer] PreCompact hook error: ${errorMessage}`);
	} else if (result.resultKind === 'success' && result.output && typeof result.output === 'object') {
	const output = result.output as PreCompactHookOutput;
	if (output.hookSpecificOutput?.additionalContext) {
	allAdditionalContext.push(output.hookSpecificOutput.additionalContext);
	}
	}
	if (result.resultKind === 'success' && result.output && typeof result.output === 'object') {
	const output = result.output as PreCompactHookOutput;
	if (output.hookSpecificOutput?.additionalContext) {
	allAdditionalContext.push(output.hookSpecificOutput.additionalContext);
	}
	}
	const warningMessage = 'warningMessage' in result && typeof result.warningMessage === 'string' ? result.warningMessage : undefined;
	const stopReason = 'stopReason' in result && typeof result.stopReason === 'string' ? result.stopReason : undefined;
	if (result.resultKind === 'error') {
	const errorMessage = typeof result.output === 'string' ? result.output : 'Unknown error';
	this.logService.error(`[ConversationHistorySummarizer] PreCompact hook error: ${errorMessage}`);
	if (warningMessage) {
	this.logService.warn(`[ConversationHistorySummarizer] PreCompact hook warning: ${warningMessage}`);
	}
	if (stopReason) {
	this.logService.warn(`[ConversationHistorySummarizer] PreCompact hook stop reason: ${stopReason}`);
	}
	} else if (result.resultKind === 'warning') {
	const resultWarningMessage = typeof result.output === 'string' ? result.output : warningMessage ?? 'Unknown warning';
	this.logService.warn(`[ConversationHistorySummarizer] PreCompact hook warning: ${resultWarningMessage}`);
	if (stopReason) {
	this.logService.warn(`[ConversationHistorySummarizer] PreCompact hook stop reason: ${stopReason}`);
	}
	}