[codex] fix Windows ConPTY input handling

[iceweasel-oai]

Why

Windows unified-exec TTY input did not behave like the non-Windows PTY path. ConPTY sessions could receive the wrong line ending or mishandle backspace, especially when sending input to a foreground program through PowerShell or cmd. The local, legacy restricted, and elevated paths also handled this normalization separately.

What changed

• share one stateful Windows TTY input normalizer across local, legacy restricted, and elevated runner paths
• translate LF and split CRLF into one Windows terminal Enter, encode backspace as DEL, and preserve UTF-8 and control bytes such as Ctrl-C
• add Windows integration coverage for Unicode input, backspace, Enter, and PowerShell foreground-child Ctrl-C behavior

Validation

• just test -p codex-utils-pty (13 tests passed; the Unicode integration test retried once)
• the Unicode integration test passed five consecutive runs with retries disabled
• integration coverage sends cafeé 漢字 through cmd and PowerShell and verifies that Ctrl-C interrupts a running PowerShell foreground child