Test the OPA adapter's validation errors and deny / not_applicable mapping #71

Checked at 3add48a.

pytest --cov=agentrust_telemetry passes (127 passed, 2 skipped), and src/agentrust_telemetry/adapters/opa.py is the least-covered module at 81%. The uncovered lines are 35, 39, 42, 45, 51, 63, 88, 90, 96 and 99.

tests/test_adapters.py:89-160 covers the allow path, the mapper requirement and malformed collections. Nothing yet tests:

• a missing decision_id, or a mapper that returns an unsupported decision
• non-object labels, or a missing version
• a negative, bool or non-integer duration, and more than 32 ids
• result: false mapping to deny and result: null mapping to not_applicable
• a non-string timestamp, or one without the Z suffix

Done when opa.py reaches 100% line coverage under the same pytest command, each error message has its own assertion, and no source file changes. cedar.py lines 30, 37 and 65 are a fair stretch goal.

Files: tests/test_adapters.py.