allow deadline alert UUID references in serialized Dag schema

[JH-A-Kim]

---

Description

This pr is a continuation of PR #68195
Related: PR #67449

What happens: When a Dag defines a deadline alert, SerializedDagModel._generate_deadline_uuids rewrites the Dag's deadline field from a list of dicts into a list of UUID strings referencing rows in the deadline_alert table. This has a few different problems.

1. schema.json only allowed deadline to be a dict, an array of dicts, or null so this would throw a validation error.

2. _generate_deadline_uuids rewrote the passed-in dag.data in place. Any of the other code that still used the reference would then get something completely different from what they expected, that being a dict of deadlines as opposed to a list of UUIDs

3. One of the possible fixes to the above using a deepcopy would then create a separate issue wherein test_dagrun_success_deadline and test_dagrun_deadline_variable_interval_stable would be using the original version of the dag data that has deadline as an actual dict instead of the changed version with deadlines as a list of strings that are UUIDs.

The fix

1. Add an array of string option to the deadline schema, so it can accept the UUID.

2. deepcopy dag.data inside write_dag before writing any of the UUID information so we prevent mutation of the caller object.

3. In _write_dag, do session.flush() to get the updated information to the db and then from there just call down from the db as the single source of truth. Important thing to note is there is no commit so only that session sees the changes and no other sessions see it. (tldr of this fix is to make it so that it acts like a client server contract with video games wherein all the important information needs to be gathered from the source of truth and not locally.)

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: Sonnet 5 following the guidelines

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[JH-A-Kim]

@ferruzzi may I have a review on this? Thank you!

[ferruzzi]

Left a couple small comments.

[ferruzzi]

Pretty sure those failing CI tests were just fixed in main, give it a rebase and they should go green.

[JH-A-Kim]

@ferruzzi i believe this is ready for review again!