DecisionCertificate.__str__ should return the human-readable summary

[raphaelrrcoelho]

certify_decision returns a DecisionCertificate (a NamedTuple) whose .summary field is a plain-English verdict, but print(cert) shows the raw tuple. Add __str__ returning self.summary so print(certify_decision(...)) reads naturally. Add a one-line test.

Files: src/causalrl/identification/decision.py, tests/test_certify_decision.py.

[Rajesh270712]

Hi, I have a focused fix ready for this. It makes DecisionCertificate.__str__ return the existing human-readable summary while preserving the tuple fields and repr(cert) behavior, with a regression assertion in tests/test_certify_decision.py. I validated it with python3 -m uv run pytest, python3 -m uv run ruff check ., python3 -m uv run ruff format --check ., and python3 -m uv run pyright src. If this approach looks aligned, could you assign this issue to @Rajesh270712?