protocol: Support complex stream protocol

[634750802]

Current Stream Protocol

Reuse stream protocol part of Vercel AI SDK

Non-StackVM Engine

[DATA]: chat and message object
[MESSAGE_ANNOTATION]: SOURCE_NODES, KG_RETRIEVAL, REFINE_QUESTION, ...
[MESSAGE_ANNOTATION]: SOURCE_NODES, KG_RETRIEVAL, REFINE_QUESTION, ...
[TEXT] Hello,
[TEXT] world.

StackVM Engine

[DATA]: chat and message object
- [MESSAGE_ANNOTATION]: SOURCE_NODES, KG_RETRIEVAL, REFINE_QUESTION, ...
+ [MESSAGE_ANNOTATION]: { state: StackVM.State }
+ [TOOL_CALL]: ...
+ [TOOL_RESULT]: ...
+ [MESSAGE_ANNOTATION]: { state: StackVM.State }
[TEXT] Hello,
[TEXT] world.
+ [FINISH_MESSAGE]

Goal

Support other states in External Engine protocol:

[DATA]: chat and message object

# We still can send old annotations before enter StackVM stream
[MESSAGE_ANNOTATION]: SOURCE_NODES, KG_RETRIEVAL, REFINE_QUESTION, ...

# From here we start to proxy external
+ [MESSAGE_ANNOTATION]: EXTERNAL_STREAM_START, { type: 'StackVM' }
+ [external.MESSAGE_ANNOTATION]: { state: StackVM.State }
+ [external.TOOL_CALL]: ...
+ [external.TOOL_RESULT]: ...
+ [external.MESSAGE_ANNOTATION]: { state: StackVM.State }
+ [external.TOOL_CALL]: ...
+ [external.TOOL_RESULT]: ...
+ [external.TEXT] Hello,
+ [external.TEXT] world.
+ [external.FINISH_MESSAGE]
# From here we stop to proxy external stream

[MESSAGE_ANNOTATION]: SOURCE_NODES, KG_RETRIEVAL, REFINE_QUESTION, ...

# Texts are generated from StackVM，no more text should be sent after the external stream
- [TEXT] Hello,
- [TEXT] world.

• External stream must conform to Vercel AI SDK's stream protocol.
• External stream must finish with a FINISH_MESSAGE part.
• All TiDB.ai clients could ignore all MESSAGE_ANNOTATION parts.
• TOOL_CALL and TOOL_RESULT can be used for tracking process of a external chat request.