Add fuzzing tests for validation

[ivanauth]

Summary

Add Go fuzz tests for ObjectID, Namespace, Relation, and CaveatName validation to help ensure the regex-based validation logic doesn't panic on arbitrary input.

Fixes #103

Changes

• Add FuzzObjectID - tests extended object ID validation
• Add FuzzNamespace - tests namespace/object type validation
• Add FuzzRelation - tests relation name validation
• Add FuzzCaveatName - tests caveat name validation

Testing

# Run seed corpus tests
go test -v ./proto/authzed/api/validation_test/... -run "Fuzz" -fuzz=none

# Run actual fuzzing (5 seconds)
go test ./proto/authzed/api/validation_test/... -fuzz=FuzzObjectID -fuzztime=5s