PyREPL: ESC key in isearch mode has ambiguous prefix delay

Bug report

Bug description:

This issue was noted from #145398. A bare Esc press in isearch mode (Ctrl-R) does not exit search. Instead, the first Esc appears to do nothing, only after pressing Esc a second time does isearch end.

To reproduce:

1. Press Ctrl-R in PyREPL to enter isearch.
2. Press Esc once to exit isearch.

• Expected: isearch ends immediately.
• Actual: nothing happens. Press Esc again, isearch ends.

This is because BaseEventQueue.push() treats \x1b (ESC) as a prefix of longer escape sequences (arrows, function keys, etc.). When a single Esc is received, the event queue holds it and waits for the next byte to decide whether it’s a standalone Esc or the start of a longer sequence. If no further byte arrives, the Esc event is never delivered, and isearch remains active.

Apart from isearch mode, this behavior affects normal mode as well, where Esc also has a delay before it is recognized.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-154731

从 BaseEventQueue.push() 以及 Lib/_pyrepl/unix_eventqueue.py 和 Lib/_pyrepl/windows_eventqueue.py 中的转义序列定义开始。在进行更改之前，检查相关 PR gh-154731 和现有的 PyREPL 事件处理。完成的标准是：单独的 Esc 会立即退出 isearch，并且在普通模式下无需当前的延迟即可被识别。