Fail a job when the gateway reports it could not start it

A start_print is acknowledged as soon as the gateway accepts it, and the
download and transfer that follow can still fail. That late result was only
logged, so the job sat in uploading until the stall reaper gave up twenty
minutes later.

The dispatcher now remembers which job each start command belonged to. A
late failure for it fails the job with the gateway's own reason in
metadata.lastDispatchError - but only while the job is still uploading, so a
job the printer has since started is left to its telemetry.

scripts/e2e-vendors.sh gains a phase where the proxy refuses the download
and the job must fail at once with that reason.

tests/late_failure.test.ts

@@ -0,0 +1,28 @@
+import { describe, expect, test } from "bun:test";
+import { rememberStart, takeStart } from "../src/dispatch/late_failure";
+
+describe("late start failures", () => {
+  test("a remembered start is returned once, then forgotten", () => {
+    rememberStart("c1", "job-1");
+    expect(takeStart("c1")).toBe("job-1");
+    expect(takeStart("c1")).toBeUndefined();
+  });
+
+  test("a command nobody started is unknown", () => {
+    expect(takeStart("never-sent")).toBeUndefined();
+  });
+
+  test("an hour-old start is dropped rather than failing a job long since resolved", () => {
+    const longAgo = Date.now() - 2 * 60 * 60 * 1000;
+    rememberStart("old", "job-old", longAgo);
+    rememberStart("new", "job-new");
+    expect(takeStart("old")).toBeUndefined();
+    expect(takeStart("new")).toBe("job-new");
+  });
+
+  test("the table is bounded", () => {
+    for (let i = 0; i < 1500; i++) rememberStart(`bulk-${i}`, `job-${i}`);
+    expect(takeStart("bulk-0")).toBeUndefined();
+    expect(takeStart("bulk-1499")).toBe("job-1499");
+  });
+});