Add invariant tests for NodeExitStatus classification

[sahilshingate01]

Summary

This PR adds invariant tests for NodeExitStatus classification.

A full audit of the node lifecycle and exit-status handling logic was
performed, and the current implementation was found to be correct and
well-designed. These tests encode the expected invariants to prevent
future regressions across platforms.

Scope

• Adds unit-level invariant tests only
• No runtime, daemon, or lifecycle logic changes
• No behavior changes

Invariants Covered

• Exit code 0 is classified as Success
• Non-zero exit codes are classified as ExitCode
• Unix signal termination is classified as Signal
• io::Error is propagated as IoError
• Unknown is unreachable for valid inputs

Validation

• All existing tests pass locally
• New tests are deterministic and platform-safe