fix: Allow .struct.with_fields inside list.eval

[cmdlineluser]

Closes #17356

df = pl.DataFrame({
    "x": [[{"a": 1, "b": 2}], [{"a": 3, "b": 4}]]
})

df.with_columns(
    pl.col.x.list.eval(pl.element().struct.with_fields(c = pl.field("a") + 10))
)    

# shape: (2, 1)
# ┌─────────────────┐
# │ x               │
# │ ---             │
# │ list[struct[3]] │
# ╞═════════════════╡
# │ [{1,2,11}]      │
# │ [{3,4,13}]      │
# └─────────────────┘

All tests still pass - but I'm not sure if there may be a reason why the flag is currently disabled.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.96%. Comparing base (0b0a914) to head (70eb548).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19617      +/-   ##
==========================================
- Coverage   79.96%   79.96%   -0.01%     
==========================================
  Files        1536     1536              
  Lines      211374   211374              
  Branches     2445     2445              
==========================================
- Hits       169034   169033       -1     
- Misses      41785    41786       +1     
  Partials      555      555              

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[ritchie46]

All tests still pass - but I'm not sure if there may be a reason why the flag is currently disabled.

I think this was overly conservative. Thanks for the fix.