fix(server-common): keep task streams open after messages

[dangzitou]

Description

Keep a task stream open when an agent emits a Message after the task lifecycle has started. A stateless Message remains a complete response and still closes its stream.

The new regression test covers WORKING Task -> Message -> INPUT_REQUIRED -> COMPLETED, including a custom message without task/context IDs as accepted by AgentEmitter.sendMessage(Message).

• Follow the CONTRIBUTING Guide.
• Use a Conventional Commits PR title.
• Ensure the affected tests pass.
• Appropriate documentation was updated.

Reproduction

On current main, the focused Java 17 regression was run three times. Every run failed after delivering only the working task and message:

expected: <4> but was: <2>

The following INPUT_REQUIRED and COMPLETED updates were lost because every Message was treated as stream-terminal.

Verification

Run with OpenJDK 17:

mvn -pl server-common -Dtest=EventConsumerTest test
mvn -pl server-common test

Results:

• EventConsumerTest: 19 tests passed
• server-common: 475 tests passed
• existing stateless-message stream termination coverage remains green

This fixes #1037

[kabir]

I believe this PR to be wrong for the same reasons as mentioned in #1040 (review)

Essentially the agents should either:

• Emit a Message. No Task is created, and is for single reply interactions
• Create a Task for long running tasks

The two are mutually exclusive