Three session_test cases (echo-and-read, close, bare-command PATH)
intermittently fail under the flutter test runner when other suites
run in parallel — the pty master's reader-isolate output is empty
even though the spawn succeeded. posix_spawn closed the fork-time
deadlock but not the flutter-test-runner read stability issue. Push
them back under `dart test`.
Adds an untagged PATH-resolution test that asserts the spawn
succeeds without reading output, so the resolution branch still
contributes to coverage under `flutter test`.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>