Fix ArgNotSet repr to use stable string instead of memory address

[necmo]

Issue: closes #65220

Summary

Add __repr__ and __str__ methods to ArgNotSet to return a stable "NOTSET" string instead of Python's default <... object at 0x...> which includes the memory address.

Problem

When ArgNotSet instances leak into serialization paths that fall through to str()/repr() (e.g., TriggerDagRunOperator's logical_date parameter appearing in serialized default_args), the output includes the memory address:

"logical_date": "<airflow.serialization.definitions.notset.ArgNotSet object at 0x7fe833fa8f20>"

Since the scheduler runs DagFileProcessor as separate subprocesses, each process allocates ArgNotSet at a different address → different serialized JSON → new dag_version on every parse cycle (~30 seconds).

Fix

class ArgNotSet:
    """Sentinel type for annotations, useful when None is not viable."""

    def __repr__(self) -> str:
        return "NOTSET"

    def __str__(self) -> str:
        return "NOTSET"

This ensures that even if ArgNotSet bypasses the proper is_arg_set() check in serialization, the resulting string is deterministic and does not cause spurious DAG version increments.

[boring-cyborg]

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