commands: a count reply is an int32

Two of the tests added alongside endSessions read the count reply's `n` as
int64. ReleaseFast does not check the union tag, so both passed there and
aborted in ReleaseSafe -- which is the whole argument for running the suite in
both, and the reason the plan makes ReleaseSafe non-optional.

175/175 in ReleaseFast and ReleaseSafe.

src/commands.zig

@@ -3157,13 +3157,13 @@ fn run_for_code(ctx: *Context, name: []const u8, value: bson.Value, extra: []con
     return null;
 }
 
-fn doc_count(ctx: *Context, coll: []const u8) !i64 {
+fn doc_count(ctx: *Context, coll: []const u8) !i32 {
     var reply = wire.Reply.init(testing.allocator);
     defer reply.deinit();
     var msg = try parse_fake_msg("count", .{ .string = coll }, &.{});
     defer msg.deinit();
     try dispatch(ctx, &msg, &reply);
-    return bson.get_pair(reply.pairs.items, "n").?.int64;
+    return bson.get_pair(reply.pairs.items, "n").?.int32;
 }
 
 test "a well-formed session id changes nothing and is not echoed" {
@@ -3191,7 +3191,7 @@ test "a well-formed session id changes nothing and is not echoed" {
     // mongod answers a well-formed lsid with exactly `{ok: 1}` and no echo,
     // measured; a driver reads only `$clusterTime` and `operationTime` back.
     try testing.expect(bson.get_pair(reply.pairs.items, "lsid") == null);
-    try testing.expectEqual(@as(i64, 1), try doc_count(&ctx, "sess"));
+    try testing.expectEqual(@as(i32, 1), try doc_count(&ctx, "sess"));
 }
 
 test "a malformed session id is refused without leaking a lock" {
@@ -3271,7 +3271,7 @@ test "a transactional write is refused rather than applied" {
         .{ .key = "startTransaction", .value = .{ .bool = true } },
         .{ .key = "autocommit", .value = .{ .bool = false } },
     })).?);
-    try testing.expectEqual(@as(i64, 0), try doc_count(&ctx, "txn"));
+    try testing.expectEqual(@as(i32, 0), try doc_count(&ctx, "txn"));
 
     // The order the checks fire in is mongod's, measured: a missing session id
     // is reported before the standalone refusal, and a bad type before both.
@@ -3289,7 +3289,7 @@ test "a transactional write is refused rather than applied" {
         .{ .key = "lsid", .value = lsid },
         .{ .key = "startTransaction", .value = .{ .bool = true } },
     })).?);
-    try testing.expectEqual(@as(i64, 0), try doc_count(&ctx, "txn"));
+    try testing.expectEqual(@as(i32, 0), try doc_count(&ctx, "txn"));
 }
 
 test "endSessions judges the array it is handed" {