feat(sdk): add IsHealthy public reachability probe for parity with Go SDK

[marythought]

Context

The Go SDK now has a public SDK.IsHealthy(ctx) (bool, error) method (opentdf/platform#3412) that lets downstream services run readiness probes through the SDK instead of open-coding http.Client.Get(healthzEndpoint). The Go version honors ctx for deadline/cancellation, supports OTEL tracing via interceptors, and uses gRPC Health v1.

This issue tracks the equivalent for the Java SDK.

Why

Cross-SDK feature parity for an idiomatic readiness probe. Without this, Java consumers have to either:

• Open-code an HTTP /healthz check (no auth interceptor, no OTEL trace propagation, easy to misconfigure)
• Skip readiness probes entirely

Proposed shape

Go signature is IsHealthy(ctx) (bool, error). Java idioms differ — possibilities:

• boolean isHealthy() throwing a checked exception (e.g. PlatformUnreachableException) on transport failure
• A typed HealthCheckResult if more granularity is wanted
• Match whatever pattern the Java SDK already uses for similar transport-failure-vs-success calls

Let the implementer pick the most natural Java idiom; this issue is about the capability, not the exact signature.

Acceptance

• Public method on the Java SDK performing a gRPC Health v1 probe
• Honors a deadline arg for cancellation
• Does not require callers to import gRPC health protobuf types
• Tested against a grpchealth-style test server
• Documented with a runnable example

[marythought]

Closing for now — deferring until there's a concrete consumer.

The Java SDK doesn't currently have grpc.health.v1.Health proto stubs generated. Adding them properly means touching sdk/buf.yaml to vendor the gRPC health protos, regenerating Connect-Kotlin stubs, and wiring a new HealthServiceClient into the Services interface. That's worth doing — but worth doing for a real ask, not on speculation.

Reopen when:

• A JVM downstream service is open-coding /healthz checks against the platform (the data-harbor pattern that drove the Go work in feat(sdk): IsHealthy(ctx) public reachability probe platform#3412), OR
• A user explicitly asks for it

Goal is parity with the Go SDK API: boolean isHealthy() throws SDKException, with a Duration overload.