feat(rs-dapi-client): make health check cross-platform (native + WASM)

[lklimek]

Issue being fixed or feature implemented

Makes the background health check from #3353 compile and run on both native (tokio) and WASM (browser) targets, enabling health checks in the JS/WASM SDK.

What was done?

rs-dapi-client/src/health_check.rs:

• Two-layer design: run_health_check() (public, takes CancellationToken) delegates to health_check_loop() (runtime-agnostic, takes impl Future<Output=()>)
• Replaced tokio::select! with futures::select_biased! + FusedFuture for cross-platform compatibility
• Single cfg gate for sleep: gloo_timers::future::sleep (WASM) vs tokio::time::sleep (native)
• OptionalCertificate type alias: Option<Certificate> on native, Option<()> on WASM
• Moved expired-ban check to top of Phase 2 loop (fixes edge case where all bans expired before idle sleep)

rs-dapi-client/src/lib.rs:

• Removed #[cfg(not(target_arch = "wasm32"))] from health_check module and HealthCheckConfig export

rs-dapi-client/Cargo.toml:

• Moved tokio-util to non-gated deps with default-features = false

rs-sdk/src/sdk.rs:

• start_health_check() now works on both platforms (tokio::spawn vs wasm_bindgen_futures::spawn_local)
• build() uses single non-gated sdk.start_health_check() call — no duplicated spawn logic
• Replaced Arc<ArcSwapOption<CancellationToken>> with Arc<Mutex<CancellationToken>> — simpler, no extra dep needed, token state is the state machine (cancelled = stopped, live = running)
• Added stop_health_check(), is_health_check_running() for runtime control
• Removed Drop impl and _health_check_handle field — lifecycle managed by CancellationToken

How Has This Been Tested?

• cargo clippy -p dash-sdk -p rs-dapi-client — clean
• cargo test -p dash-sdk --lib — 84 passed
• cargo test -p rs-dapi-client — all passed
• Existing tests cover: config defaults, cancel token stops loop, unreachable node detection, startup probe banning, empty address list, multi-node banning, probe settings validation, cancel during ban watch phase

Breaking Changes

None. Public API additions only (start_health_check, stop_health_check, is_health_check_running).

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

_{🤖 Co-authored by Claudius the Magnificent AI Agent}

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[github-actions]

✅ DashSDKFFI.xcframework built for this PR.

• Workflow run: https://github.com/dashpay/platform/actions/runs/23193398517
• Artifacts: DashSDKFFI.xcframework (folder), DashSDKFFI.xcframework.zip, xc_checksum.txt

SwiftPM (host the zip at a stable URL, then use):

.binaryTarget(
  name: "DashSDKFFI",
  url: "https://your.cdn.example/DashSDKFFI.xcframework.zip",
  checksum: "3472d896bdea8e9c095db4a686879be8aa45f50ee21ce3184ff10bb58bb6efce"
)

Xcode manual integration:

• Download 'DashSDKFFI.xcframework' artifact from the run link above.
• Drag it into your app target (Frameworks, Libraries & Embedded Content) and set Embed & Sign.
• If using the Swift wrapper package, point its binaryTarget to the xcframework location or add the package and place the xcframework at the expected path.