src: restore tty state on exit only if changed

[mcollina]

ResetStdio() unconditionally writes the terminal attributes recorded at startup back to stdin and stderr when the process exits, even when the process never touched the terminal. This clobbers changes made in the meantime by another process sharing the terminal. In node -h | less, less switches the terminal to raw mode once it receives the first byte of output, and node switches it back to cooked mode when it exits, so less ends up running with the terminal in cooked mode (see gwsw/less#834). The same mechanism is behind #35536 (backgrounded node process corrupting the shell's terminal state).

With this change the recorded attributes are only written back when node itself changed the terminal mode through setRawMode(), or when exiting because of a signal (SIGINT/SIGTERM, trap handler, watchdog timeout). The fail-safe from #24260 is therefore preserved for every program that asked node to change the terminal, including the setRawMode(true); process.stdin.destroy() case where libuv alone cannot reset the mode. Restoring the O_NONBLOCK flag and libuv's own uv_tty_reset_mode() are unchanged.

Verified with strace under a pty: node -e 'console.log(1)' | cat issues no tcsetattr() on exit anymore, and a consumer that enters raw mode after the first byte of output (like less 710) keeps raw mode after node exits. All test/pseudo-tty tests pass. The new test fails on the unpatched binary.

The behaviour change is for code that alters the terminal behind libuv's back (native addons, stty child processes with inherited stdio) and then exits normally without cleaning up: node no longer papers over that. Marking as semver-major for that reason.

Fixes: #66440
Refs: #35536
Refs: #66540

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AI generated, reviewed by me

[nodejs-github-bot]

Review requested:

• @nodejs/startup

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.49%. Comparing base (29bd740) to head (c61847d).
⚠️ Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node.cc	72.72%	2 Missing and 1 partial ⚠️
src/tty_wrap.cc	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66753      +/-   ##
==========================================
+ Coverage   90.47%   90.49%   +0.02%     
==========================================
  Files         791      797       +6     
  Lines      277245   277501     +256     
  Branches    53310    53324      +14     
==========================================
+ Hits       250834   251126     +292     
+ Misses      16811    16796      -15     
+ Partials     9600     9579      -21     

Files with missing lines	Coverage Δ
src/node_internals.h	80.35% <ø> (ø)
src/node_watchdog.cc	79.31% <ø> (ø)
src/tty_wrap.cc	81.92% <0.00%> (-1.00%)	⬇️
src/node.cc	79.60% <72.72%> (+0.35%)	⬆️

... and 53 files with indirect coverage changes

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