fix(executive-meetings): lock down attendee save payload (#221)

Why:
Task #220 added a client-only `_sid` field on attendee rows so React
DnD can identify rows. The two client save sites already enumerate
wire fields explicitly and never serialize `_sid`, but nothing
prevents a future refactor from accidentally leaking it. The server
was zod-default lenient (silently strips unknowns), so a regression
would either be silently absorbed (bad — silent contract drift) or
land in a future JSONB metadata column without anyone noticing.

What changed:
- `attendeeSchema` in artifacts/api-server/src/routes/executive-meetings.ts
  is now `.strict()`. Any unknown attendee key (including `_sid` or
  any future client-only field) is rejected with HTTP 400 instead of
  being silently stripped. The schema is reused by all three
  attendee-bearing endpoints (POST /executive-meetings,
  PATCH /executive-meetings/:id, PUT /executive-meetings/:id/attendees),
  so all three are covered by one change.

Tests:
- Added three API tests in
  artifacts/api-server/tests/executive-meetings.test.mjs:
  1. POST /executive-meetings with attendee carrying `_sid` returns
     400 and the error mentions the rejected key.
  2. PATCH /executive-meetings/:id with attendees carrying `_sid`
     returns 400 AND the meeting's existing attendee list is
     preserved (no partial mutation).
  3. PUT /executive-meetings/:id/attendees with attendee carrying
     `_sid` returns 400 AND the seeded attendee is unchanged.

Verification:
- Full API test suite: 207/207 green (was 204/204 before; +3 new).
- No client-side change needed: existing `saveAttendeeName` (~L943
  in artifacts/tx-os/src/pages/executive-meetings.tsx) and the
  manage-dialog save (~L4004) already project to the documented
  wire shape (`name, title, attendanceType, sortOrder, kind`).
- Architect review: addressed the one gap (PATCH coverage) by
  adding test #2 above; verdict resolved.

Out of scope cleanup:
- Marked the descriptions of stale tasks #172 and #179 as STALE
  (both PDF tests pass and PDF export works in current main).
  Final cancellation left to the user.

artifacts/api-server/tests/executive-meetings.test.mjs

@@ -264,6 +264,129 @@ test("Meetings: PUT /attendees replaces the attendee list", async () => {
   assert.ok(!body.attendees.find((a) => a.name === "Old One"));
 });
 
+// --- Attendee payload contract (Task #221) ---------------------------------
+// Server is `.strict()` on the attendee schema so client-only fields like
+// `_sid` (the React row id used by the drag-and-drop editor) are rejected
+// loudly with 400 instead of being silently stripped. This locks down the
+// wire contract so a future client refactor that accidentally serializes
+// `_sid` (or any other internal field) breaks tests immediately rather
+// than being absorbed by lenient parsing.
+test("Attendee payload contract: POST rejects unknown fields like `_sid`", async () => {
+  const create = await api(adminCookie, "POST", "/api/executive-meetings", {
+    titleAr: "س",
+    titleEn: "S",
+    meetingDate: today,
+    attendees: [
+      {
+        name: "Leaky One",
+        attendanceType: "internal",
+        sortOrder: 0,
+        _sid: "client-only-row-id",
+      },
+    ],
+  });
+  assert.equal(
+    create.status,
+    400,
+    `expected 400 when attendee carries _sid, got ${create.status}`,
+  );
+  const body = await create.json();
+  // Zod surfaces the rejected key in the error path.
+  const serialized = JSON.stringify(body);
+  assert.ok(
+    serialized.includes("_sid") || serialized.toLowerCase().includes("unrecognized"),
+    `expected error to mention the rejected key, got: ${serialized}`,
+  );
+});
+
+test("Attendee payload contract: PATCH /:id rejects unknown attendee fields like `_sid`", async () => {
+  const create = await api(adminCookie, "POST", "/api/executive-meetings", {
+    titleAr: "ت",
+    titleEn: "T",
+    meetingDate: today,
+    attendees: [{ name: "Patch Seed", attendanceType: "internal", sortOrder: 0 }],
+  });
+  assert.equal(create.status, 201);
+  const meeting = await create.json();
+  created.meetingIds.push(meeting.id);
+
+  const patch = await api(
+    adminCookie,
+    "PATCH",
+    `/api/executive-meetings/${meeting.id}`,
+    {
+      attendees: [
+        {
+          name: "Leaky Three",
+          attendanceType: "internal",
+          sortOrder: 0,
+          _sid: "client-only-row-id",
+        },
+      ],
+    },
+  );
+  assert.equal(
+    patch.status,
+    400,
+    `expected 400 when PATCH attendee carries _sid, got ${patch.status}`,
+  );
+
+  // The seed attendee must still be the only one stored — the rejected
+  // PATCH must not have replaced the attendee list.
+  const fetched = await api(
+    adminCookie,
+    "GET",
+    `/api/executive-meetings/${meeting.id}`,
+  );
+  const body = await fetched.json();
+  assert.equal(body.attendees.length, 1);
+  assert.equal(body.attendees[0].name, "Patch Seed");
+});
+
+test("Attendee payload contract: PUT /attendees rejects unknown fields like `_sid`", async () => {
+  const create = await api(adminCookie, "POST", "/api/executive-meetings", {
+    titleAr: "ش",
+    titleEn: "Sh",
+    meetingDate: today,
+    attendees: [{ name: "Seed", attendanceType: "internal", sortOrder: 0 }],
+  });
+  assert.equal(create.status, 201);
+  const meeting = await create.json();
+  created.meetingIds.push(meeting.id);
+
+  const replace = await api(
+    adminCookie,
+    "PUT",
+    `/api/executive-meetings/${meeting.id}/attendees`,
+    {
+      attendees: [
+        {
+          name: "Leaky Two",
+          attendanceType: "internal",
+          sortOrder: 0,
+          _sid: "client-only-row-id",
+        },
+      ],
+    },
+  );
+  assert.equal(
+    replace.status,
+    400,
+    `expected 400 when PUT attendee carries _sid, got ${replace.status}`,
+  );
+
+  // And the meeting's attendee list must not have changed (the seed row
+  // still wins, the leaky row never persists).
+  const fetched = await api(
+    adminCookie,
+    "GET",
+    `/api/executive-meetings/${meeting.id}`,
+  );
+  const body = await fetched.json();
+  assert.equal(body.attendees.length, 1);
+  assert.equal(body.attendees[0].name, "Seed");
+});
+
 test("Meetings: POST /duplicate clones a meeting onto another date", async () => {
   const create = await api(adminCookie, "POST", "/api/executive-meetings", {
     titleAr: "ب",