fix(formats): resolve nested remote requirement URLs

[tunglambk]

Summary

• resolve relative nested -r references against a remote requirements file URL
• preserve absolute nested HTTP(S) and file URLs
• keep the existing local-file path resolution unchanged
• add offline regression coverage for sibling, parent-directory, and absolute URL references

Root cause and impact

RequirementParser._parse_line() resolved every nested requirements reference with pathlib.Path. For a remote parent such as https://example.com/base/requirements.txt, this collapsed the URL into https:/example.com/base/child.txt, so importing a valid remote requirements tree failed at the first relative -r line.

The parser now uses URL joining when the parent is remote, while retaining Path joining for local files.

Fixes #3850.

Validation

• pytest -q tests/test_formats.py (56 passed)
• ruff check src/pdm/formats/requirements.py tests/test_formats.py
• ruff format --check src/pdm/formats/requirements.py tests/test_formats.py
• git diff --check

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.44%. Comparing base (95a7777) to head (b49f0dd).

Files with missing lines	Patch %	Lines
src/pdm/formats/requirements.py	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3851      +/-   ##
==========================================
+ Coverage   88.43%   88.44%   +0.01%     
==========================================
  Files         121      121              
  Lines       13262    13268       +6     
  Branches     2252     2254       +2     
==========================================
+ Hits        11728    11735       +7     
+ Misses        965      964       -1     
  Partials      569      569              

Flag	Coverage Δ
unittests	88.33% <71.42%> (+0.01%)	⬆️

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