Chronicle: fold search_subagent trajectory into parent session (#318491)

Ensures the search subagent's parentChatSessionId is always set by
falling back to the conversation's sessionId when the capturing token's
chatSessionId is unavailable. Without this fallback the search subagent
spans lacked PARENT_CHAT_SESSION_ID and were uploaded as independent
cloud sessions instead of folding into the parent.

Follow-up fix for #318463.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: digitarald

extensions/copilot/src/extension/tools/node/searchSubagentTool.ts

@@ -141,15 +141,15 @@ class SearchSubagentTool implements ICopilotTool<ISearchSubagentParams> {
 		// Create a new capturing token to group this search subagent and all its nested tool calls
 		// Similar to how DefaultIntentRequestHandler does it
 		// Pass the subAgentInvocationId so the trajectory uses this ID for explicit linking
-		const parentChatSessionId = getCurrentCapturingToken()?.chatSessionId;
+		const parentChatSessionId = getCurrentCapturingToken()?.chatSessionId ?? parentSessionId;
 		const searchSubagentToken = new CapturingToken(
 			`Search: ${options.input.query.substring(0, 50)}${options.input.query.length > 50 ? '...' : ''}`,
 			'search',
 			subAgentInvocationId,
 			'search',  // subAgentName for trajectory tracking
 			// Use invocation ID as chatSessionId so spans get their own log file
 			subAgentInvocationId,
-			// Link back to the parent session for debug log grouping
+			// Link back to the parent session for debug log grouping and cloud session folding
 			parentChatSessionId,
 			'searchSubagent',
 		);