Add per-workflow LangSmith tracing configuration docs

[mjain]

Summary

• Add documentation for the new per-workflow LangSmith tracing settings in workflow settings(Code Change PR:feat(ai): Add workflow-level LangSmith tracing override n8n#29783)
• Documents the LangSmith Credential and LangSmith Project options that allow users to configure LangSmith tracing on individual workflows, overriding instance-level environment variables

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Summary by cubic

Documented per-workflow LangSmith tracing so you can route AI node traces from specific workflows to different projects or accounts without changing global settings. Added step-by-step setup for a LangSmith API credential (stored with n8n credential encryption) and updated Workflow Settings with a LangSmith section to select a credential and project (defaults to "default"); workflow-level settings override instance env vars like LANGCHAIN_PROJECT.

^{Written for commit ec5e259. Summary will update on new commits.}

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[CLAassistant]

All committers have signed the CLA.

[cubic-dev-ai]

No issues found across 2 files

Architecture diagram

sequenceDiagram
    participant User
    participant n8nUI as n8n UI
    participant WorkflowSettings as Workflow Settings
    participant CredentialStore as Credential Store
    participant CodeChangePR as n8n Backend (from Code Change PR)
    participant LangSmithAPI as LangSmith API
    
    Note over User,LangSmithAPI: Per-Workflow LangSmith Tracing Configuration Flow
    
    User->>n8nUI: Opens workflow settings
    n8nUI->>WorkflowSettings: Display LangSmith section
    
    Note over User,CredentialStore: Setup LangSmith Credential
    
    User->>n8nUI: Go to Settings > Credentials > Add Credential
    n8nUI->>CredentialStore: Select LangSmith API credential type
    User->>n8nUI: Enter API Key from LangSmith settings
    n8nUI->>CredentialStore: Save LangSmith API credential
    CredentialStore->>CredentialStore: Encrypt and store API key
        
    Note over User,WorkflowSettings: Configure Workflow Tracing
    
    User->>WorkflowSettings: Select LangSmith Credential
    User->>WorkflowSettings: Enter LangSmith Project name (defaults to "default")
    User->>WorkflowSettings: Save workflow settings
    WorkflowSettings->>CodeChangePR: Pass per-workflow LangSmith config
    CodeChangePR->>CodeChangePR: Override instance-level env vars for this workflow
    
    alt AI Node Execution Started
        WorkflowSettings->>CodeChangePR: Trigger execution with LangSmith config
        CodeChangePR->>CredentialStore: Retrieve LangSmith API credential
        CredentialStore-->>CodeChangePR: Decrypted API key
        CodeChangePR->>LangSmithAPI: Send traces to configured project
        LangSmithAPI-->>CodeChangePR: Trace recorded
    else Instance-level env vars used (no per-workflow config)
        Note over CodeChangePR: Fallback to LANGCHAIN_PROJECT env var
    end

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[github-actions]

Thanks @mjain! We'll hold off on reviewing this until the PR is ready to go in the main repo.