[v3-0-test] Fix Jinja2 Template deep copy error with dag.test (#51670)

[github-actions]

The dag.test method currently (in 3.0.2) clears dag runs before running ( code ) the dag. For that it uses SchedulerDAG.clear_dags, which create SchedulerDAG from Task SDK DAG -- to do that it uses deepcopy.copy.

Now that works for most cases, but not where jinja2.Template class is passed as a Task/Operator argument because of this issue.

In [1]: from copy import deepcopy
   ...:
   ...: from jinja2 import Template
   ...:
   ...: deepcopy(Template(''))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[1], line 5
      1 from copy import deepcopy
      3 from jinja2 import Template
----> 5 deepcopy(Template(''))

File ~/.local/share/uv/python/cpython-3.12.8-macos-aarch64-none/lib/python3.12/copy.py:162, in deepcopy(x, memo, _nil)
    160                 y = x
    161             else:
--> 162                 y = _reconstruct(x, memo, *rv)
    164 # If is its own copy, don't memoize.
    165 if y is not x:

File ~/.local/share/uv/python/cpython-3.12.8-macos-aarch64-none/lib/python3.12/copy.py:253, in _reconstruct(x, memo, func, args, state, listiter, dictiter, deepcopy)
    251 if deep and args:
    252     args = (deepcopy(arg, memo) for arg in args)
--> 253 y = func(*args)
    254 if deep:
    255     memo[id(x)] = y

File ~/.local/share/uv/python/cpython-3.12.8-macos-aarch64-none/lib/python3.12/copyreg.py:99, in __newobj__(cls, *args)
     98 def __newobj__(cls, *args):
---> 99     return cls.__new__(cls, *args)

TypeError: Template.__new__() missing 1 required positional argument: 'source'

This is a general issue that can affect any DAG using custom arguments that store or cache Jinja2 Template objects or similar.
Clearly, the longer term solution for dag.test is to not use SchedulerDAG like that and instead call some task Execution endpoint to clear dagruns -- but this works for 3.0.x.
(cherry picked from commit 75f8aa9)

Co-authored-by: Kaxil Naik kaxilnaik@gmail.com