fix(python): Handle current position of file objects

[ruihe774]

#17315 introduced an issue that if a file-like object was passed to read_* functions, its current stream position was not correctly respected. This PR fixes it by invalidating read buffering of file-like objects and using the stream position as offset when mmapping. This allows e.g. reading a table file with custom heading:

with open("table") as f:
    schema = parse_custom_header(f.readline())
    pl.read_csv(f, schema=schema, ...)

If the file is mmapped by reading functions (i.e. in get_reader_bytes()), the stream position of the file will not be updated after reading. This PR documents this behavior. Giving that reading functions usually consume all remaining file content and file objects after reading are seldom used anymore, IMO not updating stream position is not a big problem.

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.49%. Comparing base (daf2e49) to head (2cf1a23).
Report is 2 commits behind head on main.

Files	Patch %	Lines
py-polars/src/file.rs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17543   +/-   ##
=======================================
  Coverage   80.48%   80.49%           
=======================================
  Files        1483     1483           
  Lines      195118   195196   +78     
  Branches     2778     2778           
=======================================
+ Hits       157039   157115   +76     
- Misses      37568    37570    +2     
  Partials      511      511           

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