fix(coding-agent): scoped subagent local root overrides

Keep subagent localProtocolOptions on their ToolSession instead of installing
them as the process-global LocalProtocolHandler override. No-context URL
consumers therefore retain the active top-level session's mapping while tan
and task subagents continue to resolve through their caller context.

Add SDK regression coverage proving subagent creation preserves an existing
global mapping.

Fixes #6971

(cherry picked from commit a02eef174b03036dc960c842a0901d22333ad9cd)

packages/coding-agent/src/internal-urls/local-protocol.ts

@@ -390,8 +390,9 @@ export class LocalProtocolHandler implements ProtocolHandler {
 
 	/**
 	 * Install a process-global override that wins over the AgentRegistry-based
-	 * derivation. Used by SDK consumers that wire `localProtocolOptions` on
-	 * `createAgentSession` and by subagents that share their parent's root.
+	 * derivation. Used by top-level SDK consumers that wire
+	 * `localProtocolOptions` on `createAgentSession`; subagents keep their
+	 * inherited mapping session-bound.
 	 */
 	static setOverride(value: LocalProtocolOptions | undefined): void {
 		LocalProtocolHandler.#override = value;

packages/coding-agent/src/sdk.ts

@@ -1772,7 +1772,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			getArtifactsDir,
 			getSessionId: () => sessionManager.getSessionId?.() ?? null,
 		};
-		if (options.localProtocolOptions) {
+		if (options.localProtocolOptions && !options.parentTaskPrefix) {
 			LocalProtocolHandler.setOverride(options.localProtocolOptions);
 		}
 		toolSession.getArtifactsDir = getArtifactsDir;