fix(coding-agent): skip extension-removal reporting for partial test runners

Test hosts and partial ExtensionRunner implementations do not expose
getExtensionIdentities; teardownCurrent now only captures and
reportRemovedExtensions only emits when both runners expose it, so those hosts
degrade to the previous behavior instead of throwing during session replacement.
Caught by CI's rpc-session-registry suite, which uses a minimal runner stub.

Author: code-yeongyu

packages/coding-agent/src/core/agent-session-runtime.ts

@@ -187,11 +187,15 @@ export class AgentSessionRuntime {
 
 	private async teardownCurrent(reason: SessionShutdownEvent["reason"], targetSessionFile?: string): Promise<void> {
 		const oldRunner = this.session.extensionRunner;
-		this._removedOnReplacement = {
-			oldRunner,
-			oldIdentities: oldRunner.getExtensionIdentities(),
-			reason,
-		};
+		// Test hosts and partial runner implementations may lack identity introspection;
+		// skip removal reporting there rather than break the replacement itself.
+		if (typeof oldRunner.getExtensionIdentities === "function") {
+			this._removedOnReplacement = {
+				oldRunner,
+				oldIdentities: oldRunner.getExtensionIdentities(),
+				reason,
+			};
+		}
 		await emitSessionShutdownEvent(oldRunner, {
 			type: "session_shutdown",
 			reason,
@@ -205,8 +209,10 @@ export class AgentSessionRuntime {
 		const pending = this._removedOnReplacement;
 		this._removedOnReplacement = undefined;
 		if (!pending) return;
+		const newRunner = this.session.extensionRunner;
+		if (typeof newRunner.getExtensionIdentities !== "function") return;
 		const newResolvedPaths = new Set(
-			this.session.extensionRunner.getExtensionIdentities().map((extension) => extension.resolvedPath),
+			newRunner.getExtensionIdentities().map((extension) => extension.resolvedPath),
 		);
 		const removed = pending.oldIdentities.filter((extension) => !newResolvedPaths.has(extension.resolvedPath));
 		if (removed.length === 0) return;