depr(python): Deprecate LazyFrame.fetch

[alexander-beedie]

Closes #17267.

Removes a long-standing footgun ... 👌

---

Double-checking if we want to deprecate, or if we should make it a breaking change to clean things up for the 1.0 (in which case we can just add the breaking tag and merge). Given its status as a debug-only function that already has warnings on it, this probably isn't unreasonable - by definition it can't be used in production workflows 🤔

Update: deprecation it is!

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.71%. Comparing base (d444b79) to head (cd85460).
Report is 1 commits behind head on main.

Files	Patch %	Lines
py-polars/polars/lazyframe/frame.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17278      +/-   ##
==========================================
- Coverage   80.72%   80.71%   -0.02%     
==========================================
  Files        1475     1475              
  Lines      193162   193165       +3     
  Branches     2751     2752       +1     
==========================================
- Hits       155922   155904      -18     
- Misses      36732    36753      +21     
  Partials      508      508              

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[ritchie46]

We should tag breaking, but it's fine to undraft.

[alexander-beedie]

We should tag breaking, but it's fine to undraft.

@stinodego has convinced me to deprecate first, as it takes so little effort to do so :)

[stinodego]

Thanks, a few minor remarks and this can go in!