Close #107

What this PR does

This PR fixes intermittent failures in SelectionFeatureIT and other integration tests by ensuring the terminal's write queue is flushed after each sendKeys() call.

The Problem

xterm.js processes terminal.write() calls asynchronously for performance. When our key handlers call terminal.write() with escape sequences, the buffer update happens later in the event loop. Our test assertions were sometimes running before the buffer was updated, causing flaky failures.

The Fix

Modified XTermElement.sendKeys() to use executeAsyncScript with xterm's write callback:

@Override
public void sendKeys(CharSequence... keysToSend) {
    input.sendKeys(keysToSend);
    executeAsyncScript(
        "var callback = arguments[arguments.length - 1]; " +
        "this.terminal.write('', callback);"
    );
}

The empty write('') with a callback ensures all pending writes are processed before the script returns, making the test deterministic.

Why not Thread.sleep() or waitUntil()?

• Thread.sleep(): Works but is non-deterministic (and slows down tests unnecessarily)
• waitUntil(): Doesn't help because the buffer value doesn't change between polls until the write queue is flushed. The polling reads the same stale buffer value repeatedly and there's no observable state change to wait for.

Testing

• Ran SelectionFeatureIT multiple times consecutively with no failures

Summary by CodeRabbit

• Tests
  • Expanded selection feature test suite with additional test scenarios covering text selection, cursor positioning, and deletion operations.
  • Enhanced test synchronization to ensure proper Vaadin readiness before test execution.

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