feat(core): agnostic background task UI with CompletionBehavior (#22740)

Co-authored-by: mkorwel <matt.korwel@gmail.com>

packages/core/src/config/topicState.ts

@@ -0,0 +1,48 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * Manages the current active topic title and tactical intent for a session.
+ * Hosted within the Config instance for session-scoping.
+ */
+export class TopicState {
+  private activeTopicTitle?: string;
+  private activeIntent?: string;
+
+  /**
+   * Sanitizes and sets the topic title and/or intent.
+   * @returns true if the input was valid and set, false otherwise.
+   */
+  setTopic(title?: string, intent?: string): boolean {
+    const sanitizedTitle = title?.trim().replace(/[\r\n]+/g, ' ');
+    const sanitizedIntent = intent?.trim().replace(/[\r\n]+/g, ' ');
+
+    if (!sanitizedTitle && !sanitizedIntent) return false;
+
+    if (sanitizedTitle) {
+      this.activeTopicTitle = sanitizedTitle;
+    }
+
+    if (sanitizedIntent) {
+      this.activeIntent = sanitizedIntent;
+    }
+
+    return true;
+  }
+
+  getTopic(): string | undefined {
+    return this.activeTopicTitle;
+  }
+
+  getIntent(): string | undefined {
+    return this.activeIntent;
+  }
+
+  reset(): void {
+    this.activeTopicTitle = undefined;
+    this.activeIntent = undefined;
+  }
+}