Make the HITLOperator summary extension point public

[shahar1]

related: #70345

Human Summary

I had some concerns regarding _hitl_summary_extra being a private dictionary after I merged the above.
As the docs state that subclasses can extend HITLOperator, it should function better as a public attribute.

AI Summary

#70345 turned hitl_summary into a computed property (so templated subject/body are read after rendering) and moved runtime/subclass additions into a private _hitl_summary_extra dict. That left HITLOperator subclasses without a supported way to extend the summary — the pattern the old attribute explicitly documented ("subclasses can extend it") now silently mutates a throwaway snapshot — and it leaks the private attribute name into OpenLineage include_full_task_info events (which serialize the operator's __dict__, where a property never appears).

This renames the dict to a public hitl_summary_extra and documents the contract on the property: each access builds a fresh snapshot, mutation of the returned dict has no effect, and extensions go through hitl_summary_extra. Since #70345 has not shipped in a provider release yet, the rename is free of compatibility cost — waiting until after the next provider wave would turn it into a real break.

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

• Yes — Claude Code (Fable 5)

Generated-by: Claude Code (Fable 5) following the guidelines

[potiuk]

Agreed with the concern that prompted this. Once hitl_summary became a property, each access builds a fresh dict, so the documented "subclasses can extend it" pattern silently stopped working — anything written to the returned dict lands in a throwaway snapshot. Leaving the only working route behind a private name wasn't a real extension point.

The docstring addition is the part I value most here: writing down that the snapshot is rebuilt per access is what stops the next person rediscovering this the hard way.

Checked that the rename is complete — all ten internal sites, and nothing outside hitl.py referenced the private name. No compat exposure either, since _hitl_summary_extra was private and hasn't appeared in a released changelog entry.

One thought for later, not for this PR: a public mutable dict means subclasses have to call super().__init__() before touching it or they get an AttributeError. Both current subclasses do. An overridable hook would be order-independent, but the dict matches how the internals already use it and is the smaller change.

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Drafted-by: Claude Code (Opus 5); reviewed by @potiuk before posting