Fix backfill KeyError when try_number out of sync

[awbush]

This patch ensures that backfills stop encountering KeyError when popping from running tasks.

• TaskInstanceKey.reduced is no longer referenced and could be deprecated/removed.
• Instead, TaskInstance primary key is always used to sync in-memory and DB states.
• Code comments no longer mention reduced_key and instead use in-memory key.

The issue goes back many years, but these stood out:

• related: Backfill crashes with "KeyError: TaskInstanceKey" when task has retries #13322
• related: [Bug] Backfill job fails to run when there are tasks run into rescheduling state. #17305
• related: Add unittest for #17305 #18806

Note that scheduler_job_runner.py works around this issue too.

---

^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our pre-commits will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example DAG that shows how users should use it.
• Consider using Breeze environment for testing locally, it's a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack

[potiuk]

I believe a better solution is being discussed in #30669 - I think we really need to add a new db entity to handle all problems with try_num. Closing this as essentially duplicate - and I think discussions should happen there.

[potiuk]

@hussein-awala @ashb @uranusjr -> maybe indeed (see the comment in #30669 (comment)) we could merge that one to 2.6.* to fix the backfill_job try_num calculation before we have a more permanent fix.

Need a second pair of eyes on that one.

[hussein-awala]

looks good, LGTM

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.