Add DECRQM support for synchronized output mode

Applications can now detect synchronized output support by querying
CSI ? 2026 $ p and receiving CSI ? 2026 ; <mode> $ y response.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/common/InputHandler.test.ts

@@ -2314,7 +2314,7 @@ describe('InputHandler', () => {
     });
     it('DEC privates with set/reset semantic', async () => {
       // initially reset
-      const reset = [1, 6, 9, 12, 45, 66, 1000, 1002, 1003, 1004, 1006, 1016, 47, 1047, 1049, 2004];
+      const reset = [1, 6, 9, 12, 45, 66, 1000, 1002, 1003, 1004, 1006, 1016, 47, 1047, 1049, 2004, 2026];
       for (const mode of reset) {
         await inputHandler.parseP(`\x1b[?${mode}$p`);
         assert.deepEqual(reportStack.pop(), `\x1b[?${mode};2$y`);   // initial reset

src/common/InputHandler.ts

@@ -2298,6 +2298,7 @@ export class InputHandler extends Disposable implements IInputHandler {
     if (p === 1048) return f(p, V.SET); // xterm always returns SET here
     if (p === 47 || p === 1047 || p === 1049) return f(p, b2v(active === alt));
     if (p === 2004) return f(p, b2v(dm.bracketedPasteMode));
+    if (p === 2026) return f(p, b2v(dm.synchronizedOutput));
     return f(p, V.NOT_RECOGNIZED);
   }