feat: improve error handling for connection failures

[marythought]

Recommendation: Enhance Java SDK error handling to catch connection failures and provide clearer messages. This requires changes to the SDK code itself, not documentation:

// Suggested enhancement in io.opentdf.platform.sdk.SDKBuilder
try {
 // existing connection code
} catch (StatusRuntimeException e) {
 if (e.getStatus().getCode() == Status.Code.UNAVAILABLE) {
 throw new SDKException(String.format(
 "Failed to connect to OpenTDF platform at %s. " +
 "Is the platform running? Verify with: curl -k %s/healthz",
 platformEndpoint, platformEndpoint
 ), e);
 }
 throw e;
}