[Serialization] Fix: Serialize DagParam like a normal Param.

[Desdroid]

When we use a DagParam as a kwarg to partial() of a dynamically mapped task, the DagParam will be serialized with its memory address. This leads to a DAG version inflation, as the memory address is not stable.

This is a simple repro:

from __future__ import annotations

import json

from airflow.sdk import DAG
from airflow.sdk import task
from airflow.serialization.definitions.param import SerializedParam
from airflow.serialization.serialized_objects import DagSerialization

if __name__ == "__main__":
    with DAG(dag_id="repro_dagparam") as dag:

        @task
        def add(value):
            return value

        @task
        def do(something):
            return something

        do(dag.param("some", "some_default_val"))

        add.partial(value=dag.param("p", "p_default_val")).expand(value=[1, 2, 3])

    ser = DagSerialization.to_dict(dag)
    mapped = ser["dag"]["tasks"][1]["__var"]
    print(json.dumps(mapped.get("partial_kwargs"), indent=2, default=str))
    print("ROUNDTRIP:")
    deser = DagSerialization.from_dict(ser)
    t = deser.get_task("add")
    print(type(t.partial_kwargs.get("op_kwargs")), t.partial_kwargs.get("op_kwargs"))

This PR fixes this issue by serializing the DagParam as a normal Param. This behavior matches the serialization that using the DagParam as a normal arg does (like in the do task - it ends up being serialized as a Param).

Note: I am not 100% sure that this is the correct fix as I'm not super deep in the DAG serialization code, but it seems to work.

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[Desdroid]

@ephraimbuddy @wjddn279 Since you were working on serialization in #63871 - what do you think of this fix I propose here? Does this make sense or is it wrong and some change is needed in serialize_template_field?
The DagParam gets serialized with it's memory address when in partial of DTM.

[yuseok89]

Have you tested this? Looking at the code, it seems only _default is stored and _name is dropped, and since DagParam resolves at runtime via dag_run.conf[self._name], I think this could be a problem. Or am I misreading the flow?