bug: app-server --stdio thread/message does not stream response_delta output

[guodage]

Summary

deepseek app-server --stdio advertises response_delta support, but thread/message returns accepted without streaming any real model text to stdout.

Environment

• DeepSeek-TUI version: observed on v0.8.14
• Transport: deepseek app-server --stdio
• Integration impact: third-party stdio clients cannot consume assistant output in real time

Steps to reproduce

1. Start the app server:

   deepseek app-server --stdio

2. Start a thread:

   {"jsonrpc":"2.0","id":1,"method":"thread/start","params":{"cwd":"/tmp"}}

3. Send a message:

   {"jsonrpc":"2.0","id":2,"method":"thread/message","params":{"thread_id":"thread-xxx","input":"Say hello"}}

4. Observe stdout events.

Expected behavior

Between response_start and response_end, stdout should contain one or more response_delta push events with real assistant text.

Actual behavior

The stdio stream only emits response_start and response_end, then returns a JSON-RPC result with status: "accepted". No actual assistant text is streamed.

Impact

Any third-party integration that depends on the documented stdio event stream sees an empty model response and cannot function correctly.

Additional context

I already opened PR #1141 with a fix and regression coverage for this behavior.