feat: support streaming reasoning chunks

[WiiliamC]

Description

This PR adds support for streaming reasoning/thinking chunks from OpenAI-compatible models such as vLLM Qwen thinking models.

It centralizes streaming chunk extraction so AgentFlow LLM and Agent nodes can separate visible response text from reasoning content, stream reasoning as thinking events, and avoid sending empty token events for reasoning-only chunks.

Changes

• Add extractStreamingChunkContent to extract visible text and reasoning content from LangChain/OpenAI-compatible streaming chunks.
• Preserve raw OpenAI-compatible responses in the custom OpenAI chat model so reasoning deltas can be read from raw streaming payloads.
• Update AgentFlow LLM and Agent streaming handlers to emit reasoning chunks through thinking events and store aggregated reasoning metadata.
• Add tests for reasoning-only streaming chunks and raw reasoning delta extraction.

Test

• Manually completed build verification.
• Manually completed port/runtime testing.
• Manually completed frontend testing.

before:

after:

[gemini-code-assist]

Code Review

This pull request refactors the streaming chunk content extraction logic across AgentFlow nodes by introducing a centralized extractStreamingChunkContent utility in utils.ts. This utility extracts both visible text and reasoning/thinking content from various LLM chunk formats, including OpenAI-compatible raw responses. The PR also adds corresponding unit tests for the new utility and streaming behavior, and configures ChatOpenAICustom to include raw responses. Feedback on the changes suggests improving the robustness of the array check for chunk.contentBlocks in extractStreamingChunkContent by using Array.isArray instead of checking the length property to prevent potential issues if contentBlocks is a non-array object or string.

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[gemini-code-assist]

Using chunk.contentBlocks?.length to check if contentBlocks is a valid array can be problematic if contentBlocks is a non-array object or a string (which also has a length property). It is safer and more robust to explicitly check if it is an array using Array.isArray(chunk.contentBlocks).

    const reasoningBlocks = Array.isArray(chunk.contentBlocks) ? chunk.contentBlocks : Array.isArray(chunk.content) ? chunk.content : []