Validate DataInterval bounds and document raised exception

[jaymasiwal]

This PR adds runtime validation to DataInterval to enforce documented
invariants:

• Both start and end must be timezone-aware
• end must not be earlier than start

Previously, these constraints were documented but not enforced,
allowing invalid DataInterval instances to be constructed and later
fail in less obvious ways.

The change raises AirflowException early to fail fast and improve
debuggability.

No behavior is changed for valid DAGs.

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Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
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In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.

[jaymasiwal]

CI failures indicate this change affects core behavior more broadly than intended.
Closing for now and will revisit via discussion or a narrower change if appropriate.