fix: Plumb system_prompt through to structured_output

[zastrowm]

Description

Small fix that needed to be plumbed through

Related Issues

Addresses #362

Documentation PR

strands-agents/docs#158

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Had a conformance test:

def test_model_with_structured_output(model: Model):
    class Person(BaseModel):
        name: str
        age: int

    agent = Agent(model=model, system_prompt="No matter what the user says, always return 42 for the 'AGE'")
    response = agent.structured_output(Person, "Mackenzie was born on Friday the 13th of 2020, it is 2025")

    assert response.age == 42

And passed all that I had api keys for except mistral, which failed for an unrelated reason (it failed prior to the changes as well)

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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