[v3-3-test] Fix new DAG versions from serializing a retry_policy object (#69243)

[github-actions]

• Fix non-deterministic serialization of mapped task retry_policy

A mapped operator keeps retry_policy in partial_kwargs, but the mapped-operator
serializer has no serializer for a RetryPolicy, so it fell back to str(obj) --
embedding the object's per-process memory address. That made the serialized DAG
non-deterministic: dag_hash changed on every parse, defeating write_dag's
"unchanged" check and creating a new DagVersion on nearly every DAG-processor
re-serialization (even for idle, never-run DAGs), polluting version history and
bloating the metadata DB.

Non-mapped operators already avoid this by excluding the object and storing only
a has_retry_policy flag; this makes the mapped path do the same. Runtime is
unaffected -- the worker re-parses the DAG source for the live policy.

• Also strip retry_policy from DAG default_args serialization

A RetryPolicy set via DAG default_args hit the same str(obj) fallback as the
mapped-operator case, embedding a memory address and producing a new DagVersion
on every parse. Route both the mapped partial_kwargs loop and the default_args
cleanup through a shared has-flag set (callbacks + retry_policy) so the object is
never serialized on either path.

• Add negative test for mapped task without a retry_policy

Mapped operators resolve has_retry_policy through _get_partial_kwargs_or_operator_default
(falling back to SerializedBaseOperator's default) rather than the dataclass default, so the
false case warrants its own test alongside the existing non-mapped one.

• Hoist retry_policy test imports and assert default_args has_retry_policy

Review follow-ups: move the retry_policy imports to module top (no collision, unlike
the SDK DAG import which stays local), shrink the has-flag branch comment to point at
_HAS_FLAG_FIELDS, and assert the positive side (has_retry_policy written, retry_policy
stripped) in the default_args test.
(cherry picked from commit b56771c)

Co-authored-by: Rahul Vats 43964496+vatsrahul1001@users.noreply.github.com