docs: document AWS (Assume Role) credential on Bedrock nodes

[its-imad]

Summary

• Documents the new Authentication parameter (AWS (IAM) / AWS (Assume Role)) on the AWS Bedrock Chat Model and Embeddings AWS Bedrock nodes
• Adds a note on the Embeddings node that the Model field can be set via Expression mode when bedrock:ListFoundationModels permission is missing
• Adds both Bedrock nodes to the AWS (Assume Role) supported nodes list on the credentials page

Linear ticket

https://linear.app/n8n/issue/DOC-1825/docs-document-aws-assume-role-credential-on-bedrock-nodes

Notes

⏳ pending-dev — blocked on n8n-io/n8n#28663 merging. Do not merge this PR until that feature PR is released.

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[github-actions]

No description provided.

[github-actions]

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

[cubic-dev-ai]

No issues found across 3 files

Architecture diagram

sequenceDiagram
    participant User as User (n8n UI)
    participant Node as Bedrock Node
    participant Creds as AWS (Assume Role) Credential
    participant STS as AWS STS
    participant Bedrock as AWS Bedrock API

    Note over User, Bedrock: Node Configuration & Authentication

    User->>Node: Selects Authentication Method
    
    alt NEW: AWS (Assume Role)
        Node->>Creds: Request temporary credentials
        Creds->>STS: NEW: Call AssumeRole(Role ARN)
        STS-->>Creds: Return Temp Access Key & Session Token
        Creds-->>Node: Provide temporary session
    else AWS (IAM)
        Node->>Node: Use static Access Key / Secret
    end

    Note over Node, Bedrock: Model Selection Flow

    Node->>Bedrock: CHANGED: bedrock:ListFoundationModels
    
    alt User has ListFoundationModels permission
        Bedrock-->>Node: Return model list
        Node-->>User: Display models in dropdown
    else NEW: Missing ListFoundationModels permission
        Bedrock-->>Node: 403 Forbidden
        Note over User, Node: UI shows empty dropdown
        User->>Node: NEW: Switch Model field to Expression mode
        User->>Node: NEW: Manually enter Model ID (e.g., titan-embed-text-v1)
    end

    Note over Node, Bedrock: Execution Flow

    User->>Node: Execute Workflow
    Node->>Bedrock: Invoke Model / Generate Embeddings
    Bedrock-->>Node: Inference/Vector Result
    Node-->>User: Data output

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