Do not use native obj Jinja2 env to render error emails - #22218

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(Some context for this) I’m helping @andyfcx to work out a solution to #22152.

The main problem with #22152 is that, when you set render_template_as_native_obj on a DAG, it would use a NativeEnvironment to create a Jinja2 template object, which in turn will be able to be rendered into a native object. However, the email-sending mechanism also uses the same Jinja2 environment, but this environment cannot correctly create a template to render into the email’s content (which must be a string, obviously).

So the proposed solution here is to add an additional flag on get_template_env to control what environment to return. If force_string is True, get_template_env will always create a SandboxedEnvironment (which is rendered into a string), even if render_template_as_native_obj is True. A NativeEnvironment is used only if render_template_as_native_obj is True and force_string is False.

But the implementation in the PR still needs some work. I’m also not entirely convinced this is the best solution, and any suggestions are welcomed. Also at least one test will be needed.

We've merged #22770 instead.