test(parser/protobuf): use NewTest* in tests

[coryan]

Towards #7722

[gemini-code-assist]

Code Review

This pull request refactors various unit tests in the parser package to use fluent test builders (such as NewTestField, NewTestMessage, and NewTestService) instead of raw struct literals, significantly improving test readability. The review feedback points out that several manual field-clearing operations (such as setting f.Parent = nil or createMethod.Service = nil) are redundant because the test helpers apitest.CheckMessage and apitest.CheckService already ignore these fields during comparison.