Fix mapped task immutability after clear

[ephraimbuddy]

We should be able to detect if the structure of mapped task has changed
and verify the integrity.

This PR is an attempt to do that

Closes: #23550

[ephraimbuddy]

Ready for review while I add the remaining test

[uranusjr]

This needs some docstrings and comments to explain why and when the logic is needed and what do they do.

[ephraimbuddy]

The general logic seems right to me, but the implementation is quite messy; this is probably due to verify_integrity is already messy to begin with. We should clean up the logic around here.

I have refactored the code. Please take a look #24114

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[ephraimbuddy]

Seems there's a flaky test:

tests/models/test_dagrun.py::test_mapped_literal_length_reduction_adds_removed_state: AssertionError: assert [(2, 'removed...e), (1, None)] == [(0, None), (...: 'removed'>)]
  At index 0 diff: (2, 'removed') != (0, None)
  Use -v to get the full diff
Error: Process completed with exit code 1.

Will look at it later

[norm]

It's Postgres 14 each time (whereas pg10 passes) so it's possibly some quirk in return format? Or it is exposing a bug :)