Define stable health response structure for network-node crate

[ReinaMaze]

close #338

Define stable health response structure for network-node crate

Summary

Establishes a predictable, structured health check response for the network-node library to support downstream tooling integration.

Changes

• HealthResponse struct: Defines stable response shape with four fields:

  • status: Health check status (currently "ok")
  • service_name: Identifies the service ("axionvera-network-node")
  • version: Package version from Cargo.toml
  • environment: Optional network/environment identifier

• health_status() function: Returns structured HealthResponse with optional environment parameter

• Serialization support: Full serde integration for JSON serialization/deserialization

• Comprehensive test suite: 18 tests covering:

  • Happy path scenarios (with/without environment)
  • All required fields validation
  • Multiple environment values (production, staging, development)
  • JSON serialization and deserialization
  • Response consistency and equality
  • Edge cases (empty strings, long strings)
  • Struct derives (Debug, Clone)

Testing

• All 18 unit tests pass
• cargo fmt complies with style guidelines
• cargo check verifies compilation
• cargo clippy passes linter checks

Notes

• Library-level implementation only (no HTTP server added)
• Maintains backward compatibility with existing code
• Ready for downstream integration