docs(extension): clarify LLMParsedResponseSchema validation path

[HaoChiBao]

Summary

• Replace the stale response_model TODO on LLMParsedResponseSchema with a note that AISdkClient already validates structured output through AI SDK Output.object.
• Avoid a redundant second Zod .parse() that can break transform/pipe schemas (per review feedback).

Test plan

• Confirm AISdkClient.createChatCompletion still returns objectResponse.output for structured responses
• No behavior change expected beyond comment accuracy

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Latest commit: 5bf0829

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[cubic-dev-ai]

All reported issues were addressed across 2 files

Architecture diagram

sequenceDiagram
    participant Client as LLM Client
    participant SDK as AISdkClient
    participant Model as LLM Model
    participant Schema as response_model.schema
    participant Caller as Calling code

    Note over Client,Caller: Structured LLM response flow

    Client->>SDK: createChatCompletion(prompt, response_model)
    SDK->>SDK: Build request with response_model schema
    SDK->>Model: Send completion request
    Model-->>SDK: objectResponse.output (raw)

    SDK->>Schema: Validate objectResponse.output

    alt Validation succeeds
        Schema-->>SDK: Parsed data
        SDK-->>Client: Return data + usage
    else Validation fails
        Schema-->>SDK: ZodError thrown
        SDK-->>Client: Error propagates
    end

    Client-->>Caller: Return structured result or error

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