fix: Poetry single-component tilde constraint converts to an invalid specifier

[VXNCXNX]

pdm import -f poetry crashes on a single-component tilde constraint such as foo = "~1". It is converted to ~=1, which is not a valid PEP 440 specifier:

before   pdm.formats.base.MetaConvertError:
         dependencies: Invalid specifier for foo: ~=1: Invalid specifier: '~=1'

after    dependencies = [
             "foo<2,>=1",
             "bar~=2.5",
             "baz~=1.2.3",
         ]

Poetry defines ~1 as >=1.0.0,<2.0.0, so the single-component case reuses _caret_upper_bound, which gives the same bound for this shape. Multi-component tilde keeps mapping to ~=, so ~2.5 and ~1.2.3 are untouched.

Follow-up to #3848, which fixed the caret branch and left this one. Test added in tests/test_formats.py; removing the branch brings back Invalid specifier: '~=1'. tests/test_formats.py passes apart from test_export_from_pylock_not_empty, which fails identically on an unmodified checkout. ruff check and format clean.

AI disclosure: written with Claude Code. I ran pdm import before and after and checked the mutation myself.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.49%. Comparing base (bc67b54) to head (79ddb34).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3852   +/-   ##
=======================================
  Coverage   88.49%   88.49%           
=======================================
  Files         121      121           
  Lines       13276    13279    +3     
  Branches     2255     2256    +1     
=======================================
+ Hits        11748    11751    +3     
  Misses        962      962           
  Partials      566      566           

Flag	Coverage Δ
unittests	88.38% <100.00%> (+<0.01%)	⬆️

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[frostming]

How about changing to ~=1.0, i think they are equivalent