Do not silently read a different file for a non-existent path with a suffix (fixes #328)

[gaoflow]

Problem (#328)

cdflib.cdfread.CDF() can read a CDF from an illegal path built from a legal path plus extra characters. For example, passing mydata.cdfINVALID (which does not exist) silently reads mydata.cdf instead of raising:

>>> import cdflib
>>> cdflib.cdfread.CDF('tests/testfiles/psp_fld_l2_mag_rtn_1min_20200104_v02.cdfINVALID').cdf_info().CDF
PosixPath('.../psp_fld_l2_mag_rtn_1min_20200104_v02.cdf')   # wrong file, no error

This is a data-integrity hazard: a user can get data from a file other than the one they asked for, with no indication anything went wrong. (Reported by @ErikPGJ for JUICE/RPWI files; the upper character limit they observed before an error simply corresponds to the OS path-length limit hit in Path.is_file().)

Cause

In the file branch of CDF.__init__:

if not path.is_file():
    path = path.with_suffix(".cdf")
    if not path.is_file():
        raise FileNotFoundError(f"{path} not found")

Path.with_suffix('.cdf') replaces the last suffix rather than appending one, so mydata.cdfINVALID → mydata.cdf. The fallback was presumably meant for the "user passed mydata without the .cdf extension" case, but it also rewrites any wrong-but-present extension to .cdf.

Fix

Only apply the .cdf fallback when the path has no suffix:

if not path.is_file():
    if not path.suffix:
        path = path.with_suffix(".cdf")
    if not path.is_file():
        raise FileNotFoundError(f"{path} not found")

• mydata (no suffix) → still falls back to mydata.cdf (behavior preserved).
• mydata.cdfINVALID (has a suffix) → no longer clobbered → FileNotFoundError.
• An existing mydata.cdf is found before the fallback is ever reached, so valid reads are unaffected.

(Also drops a duplicated self.file = path line in the same block.)

Tests

Adds test_nonexist_path_with_extra_suffix_errors, which asserts that a real test-file path with extra characters appended raises FileNotFoundError. It fails on main (DID NOT RAISE) and passes with the fix; tests/test_cdfread.py passes (5 passed). Verified the no-suffix fallback still resolves <name> → <name>.cdf.

Fixes #328.