[ENH] Extend Extension class test suite

[satvshr]

Metadata

• Reference Issue: fixes [ENH] Extension Module Tests (or Examples?) #1545
• New Tests Added: Yes
• Documentation Updated: No
• Change Log Entry: Add tests for extension interface contract and extension registry edge cases

Details

• What does this PR implement/fix? Explain your changes.
  This PR adds unit tests for the OpenML Extension interface and for extension registry behavior. The tests added are the 7 tests mentioned in [ENH] Extension Module Tests (or Examples?) #1545
• Why is this change necessary? What is the problem it solves?
  Previously, only the non-abstract registry helpers (get_extension_by_model, get_extension_by_flow) were covered. The abstract Extension interface itself was not tested.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.38%. Comparing base (3380bbb) to head (42cd48d).
⚠️ Report is 154 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (3380bbb) and HEAD (42cd48d). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (3380bbb)	HEAD (42cd48d)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1560       +/-   ##
===========================================
- Coverage   85.24%   63.38%   -21.86%     
===========================================
  Files          38       36        -2     
  Lines        5008     4351      -657     
===========================================
- Hits         4269     2758     -1511     
- Misses        739     1593      +854     

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