These expected JSON parse failures will currently fail for reasons _other_ than the tested behavior.

Note that I'm being a little cute here by fixing the tests that [now apparently contradict the spec](https://github.com/protocolbuffers/protocolbuffers.github.io/commit/c18c8a72de22b99eaf536eb1f961371510159714#diff-98c94e5ab37db71fd002eff0d9cb2cfae9b0058ea00b412b466312ad819029d7). I happen to think the spec is misguided and these tests are better behavior, but I _expect_ the tests will end up being updated to match the spec.

Closes #21873

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/21873 from hudlow:misleading-parse-failures edeb1298baeb027224a13897caabc1fc2c36c985
PiperOrigin-RevId: 773893058