Harden and test the tmux start wrapper

[mrfloris]

What changed

• Detect when 1MB-minecraft.sh exits immediately after tmux creates the session.
• Make successful _output okay messages return status 0.
• Validate the complete tmux session name, allow a documented safe character set, and reject extra arguments.
• Add repeatable Bash 3.2/5.x, ShellCheck, and fake-tmux tests.
• Run the wrapper test suite on both macOS and Ubuntu in GitHub Actions.

Why

This prevents false startup-success messages, ambiguous or silently truncated session names, and future regressions without launching a real tmux session or Paper server during tests.

Validation

• Resources/Server/tests/1MB-start/run-tests.sh all
• 53 passed, 0 failed, 0 skipped under Bash 3.2.57 and Bash 5.3.15.
• ShellCheck 0.11.0 passed.
• GitHub Actions workflow YAML parsed successfully.
• git diff --check passed.