Read HITLOperator subject/body into the summary lazily

[1fanwang]

subject and body are template fields, rendered after __init__ runs. hitl_summary was built in the constructor from self.subject/self.body, so it captured the un-rendered Jinja. OpenLineage reads hitl_summary when it builds the task START event, so a templated subject/body was emitted as raw {{ ... }} instead of the resolved value.

Make hitl_summary a property that reads subject/body at access time — after rendering — so listeners and the OpenLineage START event see the resolved values. Runtime and subclass additions (timeout_datetime, approved, branches_to_execute, …) go through a private _hitl_summary_extra dict that the property merges in; the summary keys are otherwise unchanged.

related: #70296

Testing Done

New test_summary_reflects_rendered_subject_body renders a templated subject/body and asserts the summary exposes the resolved values. RED on the pre-fix source, GREEN after the property:

# pre-fix: subject/body snapshotted un-rendered in __init__
assert op.hitl_summary["subject"] == "Review for 2020-01-01"
E   AssertionError: assert equals failed
E      -'Review for {{ ds }}'     +'Review for 2020-01-01'

# after: full suite
81 passed, 1 skipped

The example_openlineage_hitl_dag system DAG (all four HITL variants) drives the START/COMPLETE events end-to-end in CI and checks each event's hitl_summary carries the rendered subject. validate_operators_init.py on the operator exits 0.

---

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

• Yes (please specify the tool below)

Generated-by: GitHub Copilot CLI following the guidelines

[shahar1]

@Lee-W I'll be happy if you could take a second look