vt: add tests for write_pty and bell effect callbacks

Test that the write_pty callback receives correct DECRQM response
data and userdata, that queries are silently ignored without a
callback, and that setting null clears the callback. Test that
the bell callback fires on single and multiple BEL characters
with correct userdata, and that BEL without a callback is safe.

src/terminal/c/terminal.zig

@@ -864,6 +864,157 @@ test "grid_ref null terminal" {
     }, &out_ref));
 }
 
+test "set write_pty callback" {
+    var t: Terminal = null;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &t,
+        .{
+            .cols = 80,
+            .rows = 24,
+            .max_scrollback = 0,
+        },
+    ));
+    defer free(t);
+
+    const S = struct {
+        var last_data: ?[]u8 = null;
+        var last_userdata: ?*anyopaque = null;
+
+        fn deinit() void {
+            if (last_data) |d| testing.allocator.free(d);
+            last_data = null;
+            last_userdata = null;
+        }
+
+        fn writePty(_: Terminal, ud: ?*anyopaque, ptr: [*]const u8, len: usize) callconv(.c) void {
+            if (last_data) |d| testing.allocator.free(d);
+            last_data = testing.allocator.dupe(u8, ptr[0..len]) catch @panic("OOM");
+            last_userdata = ud;
+        }
+    };
+    defer S.deinit();
+
+    // Set userdata and write_pty callback
+    var sentinel: u8 = 42;
+    const ud: ?*anyopaque = @ptrCast(&sentinel);
+    set(t, .userdata, @ptrCast(&ud));
+    const cb: ?Effects.WritePtyFn = &S.writePty;
+    set(t, .write_pty, @ptrCast(&cb));
+
+    // DECRQM for wraparound mode (mode 7, set by default) should trigger write_pty
+    vt_write(t, "\x1B[?7$p", 6);
+    try testing.expect(S.last_data != null);
+    try testing.expectEqualStrings("\x1B[?7;1$y", S.last_data.?);
+    try testing.expectEqual(@as(?*anyopaque, @ptrCast(&sentinel)), S.last_userdata);
+}
+
+test "set write_pty without callback ignores queries" {
+    var t: Terminal = null;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &t,
+        .{
+            .cols = 80,
+            .rows = 24,
+            .max_scrollback = 0,
+        },
+    ));
+    defer free(t);
+
+    // Without setting a callback, DECRQM should be silently ignored (no crash)
+    vt_write(t, "\x1B[?7$p", 6);
+}
+
+test "set write_pty null clears callback" {
+    var t: Terminal = null;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &t,
+        .{
+            .cols = 80,
+            .rows = 24,
+            .max_scrollback = 0,
+        },
+    ));
+    defer free(t);
+
+    const S = struct {
+        var called: bool = false;
+        fn writePty(_: Terminal, _: ?*anyopaque, _: [*]const u8, _: usize) callconv(.c) void {
+            called = true;
+        }
+    };
+    S.called = false;
+
+    // Set then clear the callback
+    const cb: ?Effects.WritePtyFn = &S.writePty;
+    set(t, .write_pty, @ptrCast(&cb));
+    set(t, .write_pty, null);
+
+    vt_write(t, "\x1B[?7$p", 6);
+    try testing.expect(!S.called);
+}
+
+test "set bell callback" {
+    var t: Terminal = null;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &t,
+        .{
+            .cols = 80,
+            .rows = 24,
+            .max_scrollback = 0,
+        },
+    ));
+    defer free(t);
+
+    const S = struct {
+        var bell_count: usize = 0;
+        var last_userdata: ?*anyopaque = null;
+
+        fn bell(_: Terminal, ud: ?*anyopaque) callconv(.c) void {
+            bell_count += 1;
+            last_userdata = ud;
+        }
+    };
+    S.bell_count = 0;
+    S.last_userdata = null;
+
+    // Set userdata and bell callback
+    var sentinel: u8 = 99;
+    const ud: ?*anyopaque = @ptrCast(&sentinel);
+    set(t, .userdata, @ptrCast(&ud));
+    const cb: ?Effects.BellFn = &S.bell;
+    set(t, .bell, @ptrCast(&cb));
+
+    // Single BEL
+    vt_write(t, "\x07", 1);
+    try testing.expectEqual(@as(usize, 1), S.bell_count);
+    try testing.expectEqual(@as(?*anyopaque, @ptrCast(&sentinel)), S.last_userdata);
+
+    // Multiple BELs
+    vt_write(t, "\x07\x07", 2);
+    try testing.expectEqual(@as(usize, 3), S.bell_count);
+}
+
+test "bell without callback is silent" {
+    var t: Terminal = null;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &t,
+        .{
+            .cols = 80,
+            .rows = 24,
+            .max_scrollback = 0,
+        },
+    ));
+    defer free(t);
+
+    // BEL without a callback should not crash
+    vt_write(t, "\x07", 1);
+}
+
 test "grid_ref out of bounds" {
     var t: Terminal = null;
     try testing.expectEqual(Result.success, new(