Add integration and unit tests for monitoring JSON API endpoints

- Add typed response structs for JSON API parsing (minimal fields only)
- Add typed polling functions to replace stringly-typed JSON access
- Add prometheus_metrics_assertions module with polling helpers
- Add #[track_caller] to assert helpers for better debugging
- Add shutdown calls to all integration tests
- Use POLL_TIMEOUT constant instead of magic numbers
- Add unit tests for http_server.rs endpoints

Author: Eric Price

integration-tests/Cargo.lock

@@ -28,6 +28,8 @@ tracing = { version = "0.1.41", default-features = false }
 tracing-subscriber = { version = "0.3.19", default-features = false }
 hex = "0.4.3"
 clap = { version = "^4.5.4", features = ["derive"] }
+serde_json = "1"
+serde = { version = "1", features = ["derive"] }
 
 # Direct dependencies kept only for the embedded `mining_device` module.
 # Remove this block when removing: