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;
}