Use regular yaml parser instead of protoyaml

[dnephin]

While trying to update Go module dependencies we encountered this problem:

go: my-package imports
	github.com/akuity/grpc-gateway-client/pkg/grpc/gateway tested by
	github.com/akuity/grpc-gateway-client/pkg/grpc/gateway.test imports
	github.com/bufbuild/protoyaml-go imports
	github.com/bufbuild/protovalidate-go imports
	github.com/bufbuild/protovalidate-go/internal/evaluator imports
	github.com/bufbuild/protovalidate-go/internal/constraints imports
	buf.build/gen/go/bufbuild/protovalidate/protocolbuffers/go/buf/validate/priv: module buf.build/gen/go/bufbuild/protovalidate/protocolbuffers/go@latest found (v1.36.6-20250625184727-c923a0c2a132.1), but does not contain package buf.build/gen/go/bufbuild/protovalidate/protocolbuffers/go/buf/validate/priv

At some point the buf moduels were renamed from github.com/bufbuild to buf.build, and the go/buf/validate/priv has been removed in recent releases. This prevents anyone from updating the buf package.

This PR removes the dependency on buf.build by using the standard yaml parser for tests. I confirmed the tests still work by modifying the expected value and seeing the test fail.

[dnephin]

encoding/json may be another option. I haven't tried, but I suspect that could work for this test case.