refactor(session): centralize fallback title policy (#39890)

packages/util/test/session-title-fallback.test.ts

@@ -0,0 +1,49 @@
+import { describe, expect, test } from "bun:test"
+import { SessionTitleFallback } from "../src/session-title-fallback.js"
+
+const root = { title: undefined, time: { created: 0 } }
+const child = { ...root, parentID: "ses_parent" }
+
+describe("SessionTitleFallback", () => {
+  test("supplies timestamped compatibility titles", () => {
+    expect(SessionTitleFallback.withTimestampedFallback(root)).toBe("New session - 1970-01-01T00:00:00.000Z")
+    expect(SessionTitleFallback.withTimestampedFallback(child)).toBe("Child session - 1970-01-01T00:00:00.000Z")
+    expect(SessionTitleFallback.withTimestampedFallback({ ...root, title: "Generated title" })).toBe("Generated title")
+    expect(SessionTitleFallback.withTimestampedFallback({ ...root, title: "" })).toBe("")
+  })
+
+  test("supplies compact display labels", () => {
+    expect(SessionTitleFallback.displayLabel(root)).toBe("New session")
+    expect(SessionTitleFallback.displayLabel(child)).toBe("Child session")
+    expect(SessionTitleFallback.displayLabel({ title: "", parentID: "ses_parent" })).toBe("Child session")
+    expect(SessionTitleFallback.displayLabel({ title: "New session - 2026-07-30T18:45:03.662Z" })).toBe("New session")
+    expect(SessionTitleFallback.displayLabel({ title: "Child session - 2026-07-30T18:45:03.662Z" })).toBe(
+      "Child session",
+    )
+    expect(SessionTitleFallback.displayLabel({ title: "Generated title" })).toBe("Generated title")
+  })
+
+  test("recognizes historical fallback titles", () => {
+    expect(SessionTitleFallback.isFallbackTitle(undefined)).toBeTrue()
+    expect(SessionTitleFallback.isFallbackTitle("New session - 2026-07-30T18:45:03.662Z")).toBeTrue()
+    expect(SessionTitleFallback.isFallbackTitle("Child session - 2026-07-30T18:45:03.662Z")).toBeTrue()
+    expect(SessionTitleFallback.isFallbackTitle("")).toBeFalse()
+    expect(SessionTitleFallback.isFallbackTitle("New session - custom")).toBeFalse()
+  })
+
+  test("recognizes only the fallback associated with a session", () => {
+    expect(SessionTitleFallback.isExactRootFallback(root)).toBeTrue()
+    expect(
+      SessionTitleFallback.isExactRootFallback({ ...root, title: "New session - 1970-01-01T00:00:00.000Z" }),
+    ).toBeTrue()
+    expect(
+      SessionTitleFallback.isExactRootFallback({ ...root, title: "New session - 2099-01-01T00:00:00.000Z" }),
+    ).toBeFalse()
+    expect(
+      SessionTitleFallback.isExactRootFallback({
+        ...child,
+        title: "Child session - 1970-01-01T00:00:00.000Z",
+      }),
+    ).toBeFalse()
+  })
+})