Add unit tests for health module

[Copilot]

Adds test coverage for dragonfly-client/health/mod.rs which previously had none.

Changes

• test_health_new: Validates Health struct initialization with proper address binding
• test_health_handler: Verifies health endpoint returns Ok response

Tests follow repository conventions - no inline comments, self-documenting through clear variable names:

#[test]
fn test_health_new() {
    let addr = SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)), 8080);
    let shutdown = shutdown::Shutdown::new();
    let (shutdown_complete_tx, _shutdown_complete_rx) = mpsc::unbounded_channel();

    let health = Health::new(addr, shutdown, shutdown_complete_tx);

    assert_eq!(health.addr, addr);
}

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[chlins]

lgtm

[BraveY]

LGTM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.83%. Comparing base (eaaee3b) to head (04c04eb).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1641      +/-   ##
==========================================
+ Coverage   43.71%   43.83%   +0.11%     
==========================================
  Files          75       75              
  Lines       20955    20967      +12     
==========================================
+ Hits         9161     9191      +30     
+ Misses      11794    11776      -18     

Files with missing lines	Coverage Δ
dragonfly-client/src/health/mod.rs	56.09% <100.00%> (+56.09%)	⬆️

... and 1 file with indirect coverage changes

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