fix: Poetry caret on a 0.x version converts to the wrong upper bound

[VXNCXNX]

What's broken

pdm import -f poetry converts every caret constraint by bumping the major component, so a 0.x caret gets a far wider bound than Poetry means. The imported project then accepts releases Poetry would have refused.

Repro

attrs = "^0.2.3", tinylib = "^0.0.3", requests = "^2.28.1" in a Poetry project:

before:
dependencies = ["attrs<1.0.0,>=0.2.3", "tinylib<1.0.0,>=0.0.3", "requests<3.0.0,>=2.28.1"]

after:
dependencies = ["attrs<0.3.0,>=0.2.3", "tinylib<0.0.4,>=0.0.3", "requests<3.0.0,>=2.28.1"]

The fix

A caret allows changes that do not modify the leftmost non-zero component, so the bumped position depends on where that component is. _caret_upper_bound finds it instead of assuming the major.

Verification

test_convert_poetry_caret_constraint is parametrised over all seven rows of Poetry's own caret table, ^1.2.3 through ^0, checked against the table in their dependency-specification docs. Forcing the index back to 0 fails three of them with assertion errors.

Two assertions in test_convert_poetry had the old bounds baked in, cleo<1.0.0 for ^0.7.6 and cachecontrol<1.0.0 for ^0.12.4. Both fixture constraints are 0.x, so those were asserting the bug; they now read <0.8.0 and <0.13.0.

tests/test_formats.py passes, 53 tests. The wider run has one failure, test_create_venv_in_project[venv-True], identical on a clean checkout here because this box has no ensurepip. ruff check and ruff format --check clean.

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.43%. Comparing base (29c40ea) to head (a58848d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3848      +/-   ##
==========================================
+ Coverage   88.41%   88.43%   +0.01%     
==========================================
  Files         121      121              
  Lines       13258    13262       +4     
  Branches     2252     2252              
==========================================
+ Hits        11722    11728       +6     
+ Misses        966      965       -1     
+ Partials      570      569       -1     

Flag	Coverage Δ
unittests	88.32% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.