refactor(schedule): simplify request zone authority

packages/context/time-context/src/request-zone.ts

@@ -0,0 +1,54 @@
+/** Request-zone derivation shared by time-context rendering and Schedule tools. */
+
+import type { UserMessage } from '@deepseek-ai/dsh-llm'
+
+/** Client-zone facts derived from the user-rpc messages in one open turn. */
+export type ClientTimeZoneContext =
+  | { readonly kind: 'resolved'; readonly timeZone: string }
+  | { readonly kind: 'mixed'; readonly timeZones: string[] }
+  | { readonly kind: 'missing' }
+
+/** Read the Host-validated client zone from one ordinary user-rpc message. */
+function clientTimeZone(message: UserMessage): string | undefined {
+  const source = message.source
+  return source.kind === 'user'
+    && 'clientTimeZone' in source
+    && typeof source.clientTimeZone === 'string'
+    ? source.clientTimeZone
+    : undefined
+}
+
+/**
+ * Derive the unique, mixed, or missing client zone from entered request input.
+ * @param messages - User messages belonging to the current open turn.
+ * @returns A sorted, duplicate-free request-zone context.
+ */
+export function deriveClientTimeZoneContext(messages: readonly UserMessage[]): ClientTimeZoneContext {
+  const timeZones = [...new Set(messages.flatMap((message) => {
+    const timeZone = clientTimeZone(message)
+    return timeZone === undefined ? [] : [timeZone]
+  }))].sort()
+  const [timeZone, ...remaining] = timeZones
+  if (timeZone === undefined) return { kind: 'missing' }
+  if (remaining.length === 0) return { kind: 'resolved', timeZone }
+  return { kind: 'mixed', timeZones }
+}
+
+/**
+ * Render Session and request-zone facts for the model-visible time reading.
+ * @param sessionTimeZone - Immutable Session zone, or `undefined` for legacy Sessions.
+ * @param client - Client zones derived from the current open turn.
+ * @returns The two policy lines appended to a time-context reading.
+ */
+export function renderTimeZoneContext(
+  sessionTimeZone: string | undefined,
+  client: ClientTimeZoneContext,
+): string {
+  const session = sessionTimeZone ?? 'unavailable'
+  const request = client.kind === 'resolved'
+    ? client.timeZone
+    : client.kind === 'mixed'
+      ? `mixed ${JSON.stringify(client.timeZones)}`
+      : 'missing'
+  return `Session time zone: ${session}.\nClient time zone for this request: ${request}.`
+}