PTY: fix resource leaks and reader-isolate races (T-76)

NativePty.close() now awaits the reader-isolate spawn, kills the
child first to drive EOF on the master fd, awaits the isolate's
EOF acknowledgement, and only then closes the fd. Previously the
fd-close racing with the polling isolate left a window where the
fd number could be reused and the isolate would briefly target the
wrong file.

Both NativePty and PtySession now surface reader-isolate spawn
errors via the output stream's addError instead of silently
swallowing them.

PtySession.spawn closes the master fd on any post-receive failure,
closes parentSock in finally (was leaking on every spawn), and
kills the ptyc process if recvFd fails.

PtySession._recvFdAsync uses try/finally to close the ReceivePort
and kill the spawn isolate even when Isolate.spawn itself throws.

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

CHANGELOG.md

@@ -34,6 +34,15 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
   immediate EOF that looked indistinguishable from clean exit.
   PTY `write` loops on short writes; both `NativePty.write` and
   `PtySession.write` now throw `PtyException` on hard errors.
+- PTY teardown order fixed — kill the child first so the master
+  fd returns EOF, await the reader isolate exit, then close the
+  fd. Previously closing the fd while the isolate still polled it
+  could briefly target a reused fd. Reader isolate spawn errors
+  in both `NativePty` and `PtySession` are now surfaced via the
+  output stream instead of silently dropped. `_recvFdAsync` no
+  longer leaks the `ReceivePort` when `Isolate.spawn` throws, and
+  `PtySession.spawn` closes the master fd if any post-receive
+  step fails.
 
 ### Security