Fix DingTalk stream group replies and ack flow

[xfei6868]

• ## Summary

Improve DingTalk Stream reply behavior so group replies
stay in the original conversation, send an immediate
acknowledgment, and mention the asking user in group chats.

Changes

• Route DingTalk Stream replies back through the callback
  sessionWebhook instead of sending group replies as one-
  to-one messages
• Add an immediate Received, please wait. acknowledgment
  for incoming DingTalk Stream messages before agent
  processing
• Mention the original sender in group replies via DingTalk
  atUserIds
• Parse and preserve DingTalk Stream callback fields needed
  for reply routing and mention handling, including
  sessionWebhook, isInAtList, and sender identity
  metadata
• Add focused unit tests for DingTalk Stream callback
  parsing and session reply payload generation

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes
• Live integration tested (if applicable)

Additional verification:

• cargo test -p openfang-channels dingtalk_stream -- --nocapture passes
• Verified locally that dingtalk_stream starts and
  connects successfully
• Verified behavior manually in DingTalk Stream flow

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries

[jaberjaber23]

Reviewed and approved. This PR has merge conflicts with recent changes on main. Please rebase onto current main and we will merge it.

[xfei6868]

Reviewed and approved. This PR has merge conflicts with recent changes on main. Please rebase onto current main and we will merge it.

rebase it.