feat(llm): stream reasoning on a dedicated out-of-band event

[lin-snow]

Why

Reasoning models wrap chain-of-thought in <think>...</think>. In separated mode the LLM node strips it from the answer (#171) but then dropped it — so the chatflow "thinking" panel stays dark while streaming. This rescues that reasoning onto a dedicated out-of-band event parallel to the text path, leaving answer untouched.

Flow

flowchart TD
    M["model token stream<br/>(&lt;think&gt; may be split across chunks)"]
    M --> F["① ThinkStreamFilter.feed / finalize<br/>(reasoning.py)"]
    F -->|"FilterChunk(text, reasoning)"| S["② LLM node splits two paths<br/>_build_stream_text_events (llm/node.py)"]

    S -->|text| TXT["StreamChunkEvent<br/>selector = node/text"]
    TXT --> ANS["answer variable stream<br/>(existing logic, unchanged)"]

    S -->|reasoning| RSN["StreamReasoningEvent(chunk, is_final)<br/>no selector · out-of-band"]
    RSN --> LIFT["③ Node._dispatch lift: inject node_id<br/>(base/node.py)"]
    LIFT -->|"NodeRunReasoningChunkEvent"| EH["④ EventHandler.dispatch<br/>collect-only group, no warning spam<br/>(event_handlers.py)"]
    EH --> RSF["⑤ ResponseStreamFilter.on_event<br/>case _ passes through, never buffered into answer<br/>(response_stream.py)"]
    RSF --> DIFY["leaves graphon → dify step ②"]

    style RSN fill:#e8f0fe,stroke:#4285f4
    style LIFT fill:#e8f0fe,stroke:#4285f4
    style EH fill:#e8f0fe,stroke:#4285f4
    style RSF fill:#e8f0fe,stroke:#4285f4

Loading

The highlighted reasoning spine is the new path; the text path is its sibling and is unchanged. Carrying no selector is what lets reasoning ride ResponseStreamFilter's case _: untouched — no answer-routing logic ever touches it.

What

• Filter — ThinkStreamFilter.feed/finalize now return FilterChunk{text, reasoning} instead of str, surfacing the stripped reasoning (incl. truncated residual from an unclosed <think>) instead of dropping it. split_reasoning / extract_stream_reasoning are untouched.
• Event — new two-layer StreamReasoningEvent → NodeRunReasoningChunkEvent, mirroring the stream-chunk pair minus selector. Like NodeRunRetrieverResourceEvent, it's out-of-band telemetry (not a variable stream), so ResponseStreamFilter lets it ride case _: untouched.
• Node — separated mode emits reasoning alongside text and sends exactly one terminal is_final marker per run that produced reasoning (truncated residual if any, else empty).

Compatibility

• tagged mode emits no reasoning events — byte-for-byte unchanged.
• Terminal ModelInvokeCompletedEvent.reasoning_content is unchanged.
• Additive & independently mergeable: ignoring NodeRunReasoningChunkEvent is safe.

Tests

uv run pytest — 585 passed (added filter, node-stream, lift, and passthrough/no-warning coverage); ruff check, ruff format --check, ty check all clean.