pyyaml: extend loader fuzzer to exercise yaml.safe_load

[VivekGaddam]

This PR extends the existing PyYAML loader fuzzer to also exercise
yaml.safe_load, which is one of the most commonly used public APIs.

The change is intentionally minimal and reuses the existing fuzzer
infrastructure. The goal is to increase coverage of user-facing parsing
paths and ensure that safe_load does not crash on malformed input.

No behavior changes are intended outside of fuzzing coverage.

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[github-actions]

VivekGaddam is a new contributor to projects/pyyaml. The PR must be approved by known contributors before it can be merged. The past contributors are: hunsche, cclauss, TheShiftedBit, fmeum, catenacyber