OPA adapter raises TypeError, not ValueError, when a result mapper returns a dict or list #75

Checked at 986e87e. Found by rajnisht7 in review of #73.

src/agentrust_telemetry/adapters/opa.py:38 checks the mapper's return value with decision not in {"allow", "deny", "challenge", "not_applicable", "error"}. A set membership test needs a hashable value, so a result_mapper that returns a dict or a list raises TypeError: unhashable type instead of the adapter's own ValueError("OPA result mapper returned unsupported decision: ..."). Every other bad mapper result already gets that ValueError.

Done when

• a mapper returning {} or [] raises the same ValueError as an unsupported string
• a test in tests/test_adapters.py covers both, and fails on 986e87e

Files: src/agentrust_telemetry/adapters/opa.py, tests/test_adapters.py.