[Bug]: Execution-date constraints pass when the closed mandate omits `execution_date`

[mh-yu]

What happened?

Description

AP2 defines payment.execution_date as a constraint on when a payment may be executed. The closed Payment Mandate's execution_date must be within the open mandate's not_before / not_after window.

The Python SDK's ExecutionDateEvaluator does not enforce that window when the closed mandate omits execution_date. It reads the missing value and immediately returns no violations.

If omission is intended to mean immediate execution, the evaluator should compare a trusted effective execution time against the authorized window instead of passing automatically.

Impact

A closed Payment Mandate can satisfy an open mandate with an execution-date window while omitting execution_date.

The mandate signatures and chain may still verify, but the user's authorized execution window is not enforced before credential release or payment execution.

Steps to Reproduce

1. Review docs/ap2/payment_mandate.md:246-257. payment.execution_date requires the Payment Mandate execution date to be within not_before / not_after.

2. Review code/sdk/python/ap2/sdk/generated/payment_mandate.py:42-45. The closed mandate allows execution_date=None and describes omission as immediate execution.

3. Review code/sdk/python/ap2/sdk/constraints.py:316-345. ExecutionDateEvaluator.evaluate() returns [] when closed_mandate.execution_date is missing.

4. Review code/sdk/python/ap2/tests/constraints_tests.py:537-547. The existing test creates an open mandate with an execution window, passes a closed mandate with execution_date=None, and asserts that there are no violations.

Relevant log output

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