[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.