test: warn when OWL coherency checks are skipped

[SproutSeeds]

Part of #3301.

This stays on the narrow no-ROBOT honesty fix only:

• emit a visible PytestWarning when OWL coherency checks are skipped because robot is not on PATH
• document the no-ROBOT path as UNTESTED in the compliance README
• keep current behavior otherwise unchanged

Fresh verification on this branch:

• uv run --python 3.13 pytest tests/linkml/test_compliance/test_core_compliance.py -k 'min_max_values and owl' -q\n- uv run --python 3.13 pytest tests/linkml/test_compliance/test_boolean_slot_compliance.py -k 'any_of_streq_MV and owl' -q\n- PATH=/Volumes/Code_2TB/code/collaboration/.local-tools/linkml-robot/bin:$PATH uv run --python 3.13 pytest tests/linkml/test_compliance/test_core_compliance.py tests/linkml/test_compliance/test_boolean_slot_compliance.py -k 'owl and (min_max_values or any_of_streq_MV)' -q\n\nThe two issue-named ROBOT failures still do not reproduce here on current main + Python 3.13 + ROBOT 1.9.10, so this PR does not try to broaden into CI ROBOT coverage or a larger diagnosis change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.86%. Comparing base (5f3601b) to head (274a9ba).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3312      +/-   ##
==========================================
+ Coverage   80.24%   83.86%   +3.62%     
==========================================
  Files         150      150              
  Lines       17563    17563              
  Branches     3662     3662              
==========================================
+ Hits        14093    14730     +637     
+ Misses       2675     1991     -684     
- Partials      795      842      +47     

Flag	Coverage Δ
linkml	80.23% <ø> (ø)
runtime	80.23% <ø> (ø)

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