test(native-app): avoid chmodding timeout socket

[leno23]

Summary

• Avoid chmodding the Unix socket path in the native-app timeout regression test.
• Bind the test socket under a temporary restrictive umask so the socket is created with owner-only permissions on platforms like macOS where chmod on sockets returns EPERM.
• Keep the test focused on the intended hung-broker timeout behavior.

Closes #41

Testing

• PKG_CONFIG_PATH=/home/linuxbrew/.linuxbrew/lib/pkgconfig cargo test native_app::tests
• PKG_CONFIG_PATH=/home/linuxbrew/.linuxbrew/lib/pkgconfig cargo test native_app::tests::socket_request_times_out_when_broker_does_not_reply
• cargo fmt --check

Merge Automation

Auto-merge is enabled with squash merge. The PR is ready for review and remains blocked until the required review approval is satisfied.

AI assistance disclosure

This PR was prepared with AI assistance and manually verified with the commands above.

[pheidon]

@leno23 Thanks. This is aligned with #41 and the narrow scope is good.

Before this is reviewable, please harden the temporary umask handling with a guard/RAII pattern so the process-global umask is always restored even if the helper changes later. After that, I’ll run CI and validate the macOS timeout test path.