fix(cdp): add noop Emulation.setUserAgentOverride to prevent Playwright crash

Playwright calls Emulation.setUserAgentOverride when creating a
browser context with a custom user agent. Without this handler,
Lightpanda returns UnknownMethod which crashes the Playwright
driver.

Add a noop handler matching the existing pattern for other
Emulation methods (setDeviceMetricsOverride, setEmulatedMedia, etc.)
so the CDP handshake can proceed.

Fixes #1436

Signed-off-by: JiangNan <1394485448@qq.com>

src/cdp/domains/emulation.zig

@@ -24,6 +24,7 @@ pub fn processMessage(cmd: anytype) !void {
         setFocusEmulationEnabled,
         setDeviceMetricsOverride,
         setTouchEmulationEnabled,
+        setUserAgentOverride,
     }, cmd.input.action) orelse return error.UnknownMethod;
 
     switch (action) {
@@ -31,6 +32,7 @@ pub fn processMessage(cmd: anytype) !void {
         .setFocusEmulationEnabled => return setFocusEmulationEnabled(cmd),
         .setDeviceMetricsOverride => return setDeviceMetricsOverride(cmd),
         .setTouchEmulationEnabled => return setTouchEmulationEnabled(cmd),
+        .setUserAgentOverride => return setUserAgentOverride(cmd),
     }
 }
 
@@ -64,3 +66,8 @@ fn setDeviceMetricsOverride(cmd: anytype) !void {
 fn setTouchEmulationEnabled(cmd: anytype) !void {
     return cmd.sendResult(null, .{});
 }
+
+// TODO: noop method
+fn setUserAgentOverride(cmd: anytype) !void {
+    return cmd.sendResult(null, .{});
+}