RabbitMQ: JSONReader in the default JSON-RPC mapper never terminates on truncated input, causing DoS

## Summary `com.rabbitmq.tools.json.JSONReader.read()` never returns when its input ends inside a quoted string or a `//` line comment. Both scanners walk the input with `StringCharacterIterator.next()` but only compare against a delimiter, so once the iterator reaches `CharacterIterator.DONE` (`￿`) they loop forever. The string scanner (`string()`, line 210, `while (c != sep)`) also appends `￿` to a `StringBuilder` every iteration, so it fills the heap and throws `OutOfMemoryError`, taking down the JVM. The comment scanner (`skipWhiteSpace()`, lines 89-92, `while (c != '\n')`) pins a thread at 100% CPU with no allocation. This is reachable with a single message. `JsonRpcServer` and `JsonRpcClient` fall back to `DefaultJsonRpcMapper` whenever no mapper is passed (`JsonRpcServer.java:84` and `:114`, `JsonRpcClient.java:186`), and that mapper hands the raw message body straight to `JSONReader.read()` (`DefaultJsonRpcMapper.java:42` for the server request, `:52` for the client reply). A caller that can publish to the RPC request queue hangs the server; a malicious or MITM'd JSON-RPC service does the same to a client. ## Proof of concept Against `amqp-client` 5.36.0 from Maven Central: ```java import com.rabbitmq.tools.jsonrpc.DefaultJsonRpcMapper; public class Poc { public static void main(String[] args) { DefaultJsonRpcMapper mapper = new DefaultJsonRpcMapper(); mapper.parse("{\"method\":\"x", String.class); // unterminated string // mapper.parse("//", String.class); // unterminated // comment System.out.println("unreachable"); } } ``` `java -Xmx64m -cp amqp-client-5.36.0.jar:. Poc` throws `OutOfMemoryError: Java heap space` in about 0.1s and never prints. Swapping in the `//` line spins at 100% CPU and never returns. A well-formed body such as `{"method":"x"}` returns immediately. ## Impact Availability. One small, unauthenticated message stops a JSON-RPC endpoint: the unterminated string exhausts the heap, the unterminated comment pins a thread forever. Neither is recoverable per request - `JsonRpcServer.doCall` only catches `ClassCastException`, and an `OutOfMemoryError` affects the whole process. ## Scope and fix Only applications using the JSON-RPC-over-AMQP tooling (`com.rabbitmq.tools.jsonrpc`) with the default `DefaultJsonRpcMapper` are affected. `DefaultJsonRpcMapper` and `JSONReader` are deprecated in favour of `JacksonJsonRpcMapper`, but both still ship and remain the default when no mapper is supplied. The fix is to stop both loops at `CharacterIterator.DONE`.