fix(python): Allow DataTypeExpr in pl.lit()

[Kevin-Patyk]

Resolves #26644.

Implemented the fix on the Python side since the Rust function pub fn lit(...) in crates/polars-python/src/functions/lazy.rs does not contain an equivalent dtype argument.

Used DataTypeExpr.collect_dtype() to resolve the DataTypeExpr into a concrete DataType prior to running the rest of the function. Passed an empty schema ({}) to .collect_dtype() since it seems like it can resolve itself (there were many examples of just passing {} to .collect_dtype() in the method docstrings located in Class DataTypeExpr).

[coastalwhite]

I think this is the wrong solution and would break on even the most trivial of DataTypeExprs and the cases where you would actually want to use it. For example:

pl.lit(1, pl.dtype_of(pl.col.a))

I laid out some possible solutions in #26644.

[Kevin-Patyk]

@coastalwhite Thanks! Will check out the issue. Wasn't sure if this was the proper fix myself

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.74%. Comparing base (032a765) to head (8862d3d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26740      +/-   ##
==========================================
+ Coverage   81.72%   81.74%   +0.01%     
==========================================
  Files        1806     1806              
  Lines      248702   248705       +3     
  Branches     3132     3133       +1     
==========================================
+ Hits       203257   203295      +38     
+ Misses      44640    44605      -35     
  Partials      805      805              

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[nameexhaustion]

IIUC this used to work with a cast - can we just add this casting back when we get a DataTypeExpr?

[Kevin-Patyk]

Updated it to call return lit(value).cast(DataTypeExpr). Also rebased 👍