fix(envelope): accept timestamp 0 in formatEnvelopeTimestamp (#106407)

* fix(envelope): accept timestamp 0 in formatEnvelopeTimestamp

formatEnvelopeTimestamp used if (!ts) to check for missing timestamps,
which incorrectly rejected the valid numeric timestamp 0 (Unix epoch).
Replace the falsy check with an explicit undefined/null check so 0 is
treated as a valid timestamp value.

* refactor(auto-reply): tighten epoch timestamp fix

Exercise the public envelope path with an exact epoch result and keep the missing-value guard aligned with its declared input type.

Co-authored-by: zenglingbiao <zeng.lingbiao@xydigit.com>

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/auto-reply/envelope.test.ts

@@ -79,6 +79,16 @@ describe("formatAgentEnvelope", () => {
     const body = formatAgentEnvelope({ channel: "Telegram", body: "hi" });
     expect(body).toBe("[Telegram] hi");
   });
+
+  it("formats the Unix epoch timestamp", () => {
+    const body = formatAgentEnvelope({
+      channel: "WebChat",
+      timestamp: 0,
+      envelope: { timezone: "utc" },
+      body: "hello",
+    });
+    expect(body).toBe("[WebChat Thu 1970-01-01T00:00:00Z] hello");
+  });
 });
 
 describe("formatInboundEnvelope", () => {

src/auto-reply/envelope.ts

@@ -114,7 +114,7 @@ export function formatEnvelopeTimestamp(
   ts: number | Date | undefined,
   options?: EnvelopeFormatOptions,
 ): string | undefined {
-  if (!ts) {
+  if (ts === undefined) {
     return undefined;
   }
   const resolved = normalizeEnvelopeOptions(options);