fix(versions): name the version part that could not be read

[chiruu12]

Fixes #3880

The catch-all else in Version.__init__ reported every part it could not read as a postrelease. An epoch, a local version, a dev release and a plain typo all arrived there and all got the same answer, which points the reader at a segment their string does not contain.

The message now names the part that failed and lists what the parser accepts:

1!1.0: '1!1' is not a valid part of a python version specifier.
Expected a number, a trailing '*', or a trailing prerelease such as 'a1', 'b2' or 'rc1'.

Nothing about what parses changes. Only the exception text moved. InvalidPyVersion is still what gets raised, so the two handlers in models/candidates.py are unaffected.

Six parametrized cases cover the misreported forms, plus 1.0.post1 so the one accurate use of the old wording stays pinned. All six fail on the parent commit. tests/models/: 228 passed, and ruff check and ruff format --check are clean.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.81%. Comparing base (f1bb362) to head (1c65681).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3881   +/-   ##
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  Coverage   88.81%   88.81%           
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  Files         120      120           
  Lines       13357    13357           
  Branches     2271     2271           
=======================================
  Hits        11863    11863           
  Misses        931      931           
  Partials      563      563           

Flag	Coverage Δ
unittests	88.70% <ø> (ø)

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