Use sentinel to mark dag as removed on reserialization

[dstandish]

When we pass a task over RPC call we can't include the dag as part of the task without running into recursion trouble. So we just "elide" the dag from the task object, require that it be passed separately and then get reattatched to the task after deserialization. We introduce a sentinel so that way we can raise a helpful error if someone tries to access the dag attr after it's been elided. Additionally, the sentinel lets us short circuit some logic in the task.dag setter that is not helpful.

The motivator for this PR is avoiding conditional code like this:

    # when taking task over RPC, we need to add the dag back
    if isinstance(task, MappedOperator):
        if not task.dag:
            task.dag = dag
    elif not task._dag:
        task._dag = dag

And the idea for this approach came from a discussion with @uranusjr .

The addition of method get_relevant_upstream_map_indexes to the pydantic TI model is just a driveby addition and i can split it out if necessary.

[dstandish]

@uranusjr here's the task.dag sentinel PR

[dstandish]

darn, realized that this needs to be merged first: #39259

(i can't cherry pick the sentinel bit into independent PR just yet)

[jscheffl]

Looks good for me from general approach but still one Pytest failing.

[dstandish]

ok pushed some test fixes let's see 🤞

[dstandish]

green @jscheffl

[jscheffl]

For me this looks all reasonable and green. After the term change (I dislike Elided as well :-D) I would rate for LGTM.

[dstandish]

@potiuk let us know if this gets your blessing, when you have a moment.

[potiuk]

Perfect. May it be merged.