GHSA-x32g-jvvm-4725 - A2A Java SDK: Push notification credentials logged in cleartext on protobuf parse failure

### *Impact.** Push notification bearer tokens and authentication credentials submitted by clients are written to server log files in cleartext. Any operator, log aggregation system, or attacker with log access can read these secrets and use them to authenticate to the client's push notification endpoints, potentially sending forged task status updates. **Where.** `transport/rest/src/main/java/org/a2aproject/sdk/transport/rest/handler/RestHandler.java:676` in `parseRequestBody` **What.** The `parseRequestBody` method logs the entire JSON request body at `SEVERE` level when `JsonFormat.parser().merge()` throws `InvalidProtocolBufferException`. This method is called from `createTaskPushNotificationConfiguration` (line 377), where the body contains client-supplied push notification bearer tokens (field `token`) and authentication credentials (field `authentication.credentials`). These secrets are written to server log files in full. **Exploit scenario.** A client creates a push notification configuration by POSTing to `/v1/tenants/{tenant}/tasks/{taskId}/pushNotificationConfigs` with a body containing `{"url": "https://client.example.com/webhook", "token": "secret-bearer-token", "authentication": {"scheme": "Bearer", "credentials": "oauth-access-token"}, "extraField": "value"}`. The extra field causes `JsonFormat.parser().merge()` to throw `InvalidProtocolBufferException`. The entire body including `secret-bearer-token` and `oauth-access-token` is logged at SEVERE level. An attacker with log access retrieves the cleartext credentials. **Preconditions.** - Attacker must have access to server log files (centralized logging, log aggregation, shared infrastructure, or insider access) - Push notification capability must be enabled - A request body must fail protobuf deserialization while being valid JSON (unknown fields with strict parsing, schema mismatches) **Fix.** Do not log the raw request body. Log only a sanitized summary — body length and content type, not cont