fix(core): Don't include last human message twice on Chat hub message regeneration

[Cadiac]

Summary

When user regenerated messages on Chat hub we accidentally included the last human message both oh the chat history and as a new human message triggering that turn.

Instead don't include the human message on the history, as it gets replayed which inserts it into the memory again.

Related Linear tickets, Github issues, and Community forum posts

https://linear.app/n8n/issue/CHA-83/bug-duplicate-user-message-sent-on-regeneration

Review / Merge checklist

• PR title and summary are descriptive. (conventions)
• Docs updated or follow-up ticket created.
• Tests included.
• PR Labeled with release/backport (if the PR is an urgent fix that needs to be backported)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[cubic-dev-ai]

No issues found across 4 files

Architecture diagram

sequenceDiagram
    participant UI as Chat UI
    participant CHS as ChatHubService
    participant DB as Message Repository
    participant WES as WorkflowExecutionService
    participant MN as Restore Chat Memory Node
    participant CT as Chat Trigger Node

    Note over UI,CT: Message Regeneration Flow

    UI->>CHS: regenerateAIMessage(retryId)
    CHS->>DB: getManyBySessionId(sessionId)
    DB-->>CHS: full conversation history
    
    CHS->>CHS: Identify target AI message & preceding human message
    
    Note right of CHS: CHANGED: history.splice(lastHumanMessageIndex)<br/>removes the trigger message from history<br/>to prevent double-injection.

    CHS->>WES: executeChatWorkflow(workflow, executionData)
    
    rect rgb(20, 30, 50)
        Note over WES,CT: Workflow Execution Context
        WES->>MN: NEW: Inject truncated history (excluding last human message)
        WES->>CT: Inject last human message as "current" trigger input
    end

    CT->>WES: Continue workflow turn...
    WES-->>CHS: executionId & stream
    CHS-->>UI: stream response tokens
    
    Note over CHS,DB: Post-Execution
    CHS->>DB: Save new AI response

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[autologie]

Thank you