docs: document server-side heartbeat configuration and changelog entry

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## [Unreleased]
+
+- Send SSE heartbeat comments on streaming endpoints (`message:stream`, `tasks/{id}:subscribe`) at a configurable
+  interval (`a2at.server.heartbeat-interval-seconds`, default 15 seconds, `0` disables). Keeps intermediaries
+  from idle-timing-out long-running SSE connections during agent execution; the heartbeat integrates with the
+  emitter lifecycle and stops on stream completion, error, timeout, or client disconnect.
+
 ## [0.0.10] — 2026-09-16
 
 - Upgrade the A2A-T SDK dependency to 1.1.1 (patch release, no breaking API changes).

docs/en/API_REFERENCE.md

@@ -704,6 +704,7 @@ Configuration properties prefixed with `a2at.server`:
 | `a2at.server.executor-max-size`              | `8`                        | Server executor maximum threads                                        |
 | `a2at.server.executor-queue-capacity`        | `100`                      | Bounded server executor queue capacity                                 |
 | `a2at.server.executor-keep-alive-seconds`    | `60`                       | Non-core thread keep-alive in seconds                                  |
+| `a2at.server.heartbeat-interval-seconds`     | `15`                       | SSE heartbeat comment interval on streaming endpoints. Keeps intermediaries from idle-timing-out long-running SSE connections. `0` disables |
 
 ```yaml
 a2at:
@@ -716,6 +717,7 @@ a2at:
     executor-core-size: 8
     executor-max-size: 16
     executor-queue-capacity: 200
+    heartbeat-interval-seconds: 15
 ```
 
 ### A2AAutoConfiguration

docs/zh/API_REFERENCE.md

@@ -657,6 +657,7 @@ getReceivedMessages() 是保留层级的响应来源，getOutputs() 为便利投
 | `a2at.server.executor-max-size`              | `8`                        | 服务端执行器最大线程数                                   |
 | `a2at.server.executor-queue-capacity`        | `100`                      | 服务端执行器有界队列容量                                 |
 | `a2at.server.executor-keep-alive-seconds`    | `60`                       | 非核心线程存活时间（秒）                                 |
+| `a2at.server.heartbeat-interval-seconds`     | `15`                       | SSE 心跳注释间隔（秒）。防止网关在 Agent 长时间执行期间因空闲超时掐断流式连接。设为 `0` 禁用 |
 
 ```yaml
 a2at:
@@ -669,6 +670,7 @@ a2at:
     executor-core-size: 8
     executor-max-size: 16
     executor-queue-capacity: 200
+    heartbeat-interval-seconds: 15
 ```
 
 ### A2AAutoConfiguration