fix(ext-tester): set the process exit code (main's return value is ignored)

[ZhuchkaTriplesix]

Second hotfix for the 0.4.19 Release run. After #1484 Build macOS and the tester compile pass; what is left is the smoke test of querya-ext-tester (https://github.com/QueryaHub/Querya-Desktop/actions/runs/38150816601, jobs Build querya-ext-tester on all platforms).

Cause

bin/querya_ext_tester.dart declares Future<int> main(...) and returns 0 / 1 / 2, but a Dart program ignores the return value of main: every run exits 0. The workflow's smoke test (an empty directory has no manifest.json: the run must fail with exit code 1) correctly failed on that. It also means the tester has never failed a CI pipeline of an extension author, so it is a real bug of the shipped tool, not just of the smoke test.

Fix

main assigns exitCode = await _run(args).

Checked locally

Built with scripts/ci/build_ext_tester.sh and ran the binary: an empty directory exits 1, --help exits 0, no arguments exit 2. The MCP smoke commands of the same step (--help, initialize without an endpoint file, exit 1 and the message) also pass locally.

Cut from 684ee7b2 (the commit the 0.4.19 tag points at), so pubspec.yaml is still 0.4.19+2.