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

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

@@ -0,0 +1,36 @@
+export * as SessionTitleFallback from "./session-title-fallback.js"
+
+const pattern = /^(New session|Child session) - \d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z$/
+
+interface Info {
+  readonly title?: string
+  readonly parentID?: string
+  readonly time: {
+    readonly created: number
+  }
+}
+
+/** Supplies the timestamped title required by compatibility surfaces. */
+export function withTimestampedFallback(info: Info) {
+  return info.title ?? fallback(info)
+}
+
+/** Supplies a compact human label and collapses historical timestamped fallbacks. */
+export function displayLabel(info: Pick<Info, "title" | "parentID">) {
+  if (!info.title) return info.parentID ? "Child session" : "New session"
+  return info.title.match(pattern)?.[1] ?? info.title
+}
+
+/** Recognizes missing and historical root or child fallback titles. */
+export function isFallbackTitle(title?: string) {
+  return title === undefined || pattern.test(title)
+}
+
+/** Recognizes a missing title or the exact root fallback for this session. */
+export function isExactRootFallback(info: Pick<Info, "title" | "time">) {
+  return info.title === undefined || info.title === fallback({ time: info.time })
+}
+
+function fallback(info: Pick<Info, "parentID" | "time">) {
+  return `${info.parentID ? "Child" : "New"} session - ${new Date(info.time.created).toISOString()}`
+}