[uranusjr]

There are a few things that needed fixing:

1. template_fields_renderers needed to be serialised for MappedOperator so the web view can render the fields correctly.
2. A bug was accidentally introduced in Implement mapped value unpacking #21641 that caused render_templates() to always be called by TaskInstance.get_rendered_template_fields(). This is now fixed.
3. To allow templated fields to be rendered on the correct operator instance (the unmapped one), unmap() needs to support serialised operators. This is conceptually impossible, but can be approximated by unmapping into a SerializedBaseOperator and copy all the attributes from MappedOperator to it. This is good enough for cases where task instance fields are pre-rendered, since RenderedTaskInstanceFields would be able to re-populate the real attributes; for fields not yet rendered (i.e. from a DAG that’s never run), /rendered-templates has always been inaccurate, so it’s not that big a deal to not able to correctly populate all the fields (and it’s impossible anyway).
4. Another consequence on supporting unmap() is that we need to get rid of its side effect of replacing the mapped task with an unmapped one in the DAG, because the webserver won’t be able to recover from this replacement and would lose all operator-mapping information if we do that. Fortunately, I believe the task replacement is not needed in the first place, and can be avoided entirely without consequences. Furthermore, we already have a mechanism for unmapping a task without adding it to the DAG (the task validation API); we just need to rename the argument and attribute to reflect its broader usage.

Note the clicking on the Rendered button still does not work after this PR, because the button is currently missing a correct map_index argument. We need to tweak the modal to be able to display a mapped task correctly. cc @bbovenzi

[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.