fix(relay): never call a reserve cell's hosts stranded when it goes existing-only (#30609)

A reserve cell keeps no activity rows and its ledger stamps last_activity_at, so the
stranded rule read its connected hosts as unserved and re-placed them.

cloud/apps/relay/src/assignment-reserve-mode-census.test.ts

@@ -228,6 +228,27 @@ describe('reserve-mode census', () => {
     await restore()
   })
 
+  it('keeps a connected host pinned when its reserve cell goes existing-only after a ledger write', async () => {
+    let now = 1_000
+    const store = await setup(() => now)
+    const identity = { userId: 'user-a', relayHostId: 'host000000000001' }
+    const first = await store.assign(identity)
+    await setReserve(store, [first.cellId])
+    // The reserve ledger stamps last_activity_at and the cell keeps no activity rows.
+    now += 5 * 60_000
+    await database!.query('DELETE FROM relay_assignment_activity_leases WHERE user_id = ?', [identity.userId])
+    await database!.query('UPDATE relay_assignments SET last_activity_at = ? WHERE user_id = ?', [now, identity.userId])
+    await store.setCellAdmissionState(first.cellId, 'existing-only')
+    now += 2 * 60_000
+    expect(await store.assign(identity)).toMatchObject({
+      cellId: first.cellId,
+      assignmentEpoch: first.assignmentEpoch
+    })
+    const { unknown, restore } = await unknownSet(() => now)
+    expect((await unknown.assign(identity)).assignmentEpoch).toBe(first.assignmentEpoch)
+    await restore()
+  })
+
   // Only the first reserve-set read is held; it is released (or failed) by the test.
   function holdFirstRead(options: { fail?: boolean } = {}) {
     const query = database!.query.bind(database!)

cloud/apps/relay/src/assignment-store.ts

@@ -1290,6 +1290,11 @@ export class RelayAssignmentStore {
     if (state !== 'existing-only') {
       return false
     }
+    // A reserve cell keeps no activity rows and its ledger stamps last_activity_at, so zero
+    // leases proves nothing; a dead one is left to the dead-reserve path and its fence.
+    if (await this.mayBeReserveCell(text(existing, 'cell_id'), transaction)) {
+      return false
+    }
     const liveLeases = (
       await transaction.query(
         `SELECT COUNT(*) AS live FROM relay_assignment_activity_leases