Resolve Client.Java-001..005 code-review findings

Client.Java-001: redactApiKey echoed the last 4 secret characters. It now
keeps only the non-secret mxgw_<key-id>_ prefix plus ***; non-gateway-shaped
tokens return <redacted>.

Client.Java-002: a close() after a queue-overflow could wipe the enqueued
overflow exception. Terminal transitions are now serialized through a single
guarded terminate() — first terminal condition wins.

Client.Java-003: openSession never read gateway_protocol_version. Both
openSession paths now call ensureGatewayProtocolCompatible, rejecting a
non-zero mismatch and accepting unset (0) for older gateways.

Client.Java-004: register/addItem/addItem2 fell back to a return_value that
silently yields 0 when unset. The fallback is now guarded by hasReturnValue()
and throws on a protocol violation.

Client.Java-005: close() in try-with-resources could mask the body exception
when the CloseSession RPC failed. close() now catches and logs the
close-time failure; closeRaw() still surfaces it for callers that want it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

clients/java/README.md

@@ -62,6 +62,18 @@ underlying protobuf messages. `MxGatewayCommandException` and
 `MxAccessException` preserve the raw `MxCommandReply` when the gateway returns a
 data-bearing MXAccess failure.
 
+`openSession` verifies the gateway's reported `gateway_protocol_version` against
+the version this client was generated for and throws `MxGatewayException` on a
+mismatch, so an incompatible client fails fast with a clear message instead of
+issuing commands that fail downstream. A gateway that does not populate the
+field is accepted unchanged.
+
+`MxGatewaySession` implements `AutoCloseable`. The try-with-resources `close()`
+performs a `CloseSession` network RPC but swallows (and logs) any failure of
+that RPC so a close-time error never replaces the exception a try-with-resources
+body is already propagating. Call `closeRaw()` explicitly when you need to
+observe the close result or handle a close-time failure.
+
 `MxEventStream` implements `Iterator<MxEvent>` and `AutoCloseable`. Closing it
 cancels the underlying gRPC stream. Canceling or timing out a Java client call
 only stops the client from waiting; it does not abort an in-flight MXAccess COM