fix(models): never convert a file onto the file being read

[devgtv]

Summary

BaseLocalFile.to_parquet streams rows out of self.path and writes them with pq.ParquetWriter(output_path, ...), which opens its destination with "wb". When output_path resolved to self.path, the source was truncated to zero length part-way through the very read serving it, and the Parquet bytes then replaced it:

>>> csv = await ExtensionFactory.instantiate("data.csv")
>>> await csv.to_parquet(output_path="data.csv")   # exits 0
>>> open("data.csv", "rb").read(4)
b'PAR1'

The input file was silently destroyed and replaced by an unreadable one.

to_parquet() with no arguments on a Parquet source hits this as well, since self.path.with_suffix(".parquet") is the source path when the source is already .parquet.

DBF, DBC and Zip already guard against this by short-circuiting when the output exists (pysus/api/extensions.py:538, :687). This adds the same guard to the base implementation, before anything is written.

Fix

• a Parquet source is returned unchanged — it needs no conversion, and download_to_parquet already anticipates this outcome with its original_path != parquet_file.path check (pysus/api/client.py:596)
• any other source raises ConversionError naming a usable output_path
• the check compares against the resolved path, so sub/../data.csv is caught too

Tests

Added 6 tests: CSV in place raises and leaves the file byte-identical, JSON in place likewise, a convoluted-but-equivalent path is caught, a Parquet source is a no-op returning the same object, the default .csv → .parquet conversion still works, and the source survives it.

4 of the 6 fail against the previous code.

• 1739 passed, 6 skipped
• black and isort clean

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Additional details and impacted files

@@           Coverage Diff           @@
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  Coverage        ?   97.19%           
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  Files           ?      180           
  Lines           ?    23351           
  Branches        ?        0           
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  Hits            ?    22697           
  Misses          ?      654           
  Partials        ?        0           

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