Rename StepFunctionStartExecutionOperator's input field to match its constructor argument

[bramhanandlingala]

Fixes the amazon provider's StepFunctionStartExecutionOperator entry from the #70296 exemption-list burn-down.

state_machine_input is a template field, but __init__ assigned it to self.input — a name that doesn't match either the constructor argument (state_machine_input) or the declared template field, so the validate-operators-init prek hook flagged it as a missing/invalid assignment.

Renamed the internal attribute (and the corresponding entry in template_fields) from input to state_machine_input so it matches the constructor argument directly. No transformation or validation was involved — this was a pure naming mismatch, not a rendering-order bug — and the public constructor signature is unchanged, so this isn't a breaking change for existing DAGs.

• Renamed self.input → self.state_machine_input in __init__ and execute().
• Updated template_fields accordingly.
• Updated the one test assertion that referenced op.input.
• Removed the operator's entry from validate_operators_init_exemptions.txt.

Verified locally that scripts/ci/prek/validate_operators_init.py reports zero findings for this class after the change, and that the existing unit tests in test_step_function.py pass unmodified aside from the one updated attribute assertion.

Related to #70296

Gen-AI disclosure: I used a generative AI tool to help identify the root
cause, write tests, and draft the PR description. I reviewed, tested, and
verified all changes locally before submitting.

Was generative AI tooling used to co-author this PR?

• Yes [Claude]

Generated-by: Claude following the guidelines

[github-actions]

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Status	Branch	Result
❌	v3-3-test

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