update: Expand test suite for multipart subscriptions

[calvincestari]

• Expands the multipart subscriptions test suite to cover all edge cases detailed in https://github.com/apollographql/team-prometheus/pull/261.
• Refactors the parsing logic to handle null errors.

[calvincestari]

@AnthonyMDev, this PR is ready for review. There are failing tests in the 'Codegen Test Configurations' job but that's failing due to warnings about unused variables. This PR is fine to merge with those because it's merging into feature/defer-execution-networking and those warnings are fixed in #217 which also merges into feature/defer-execution-networking. So once this PR is merged I'll rebase the branch for #217.

[AnthonyMDev]

LGTM, just a question on a few tests!

[calvincestari]

There are failing tests in the 'Codegen Test Configurations' job but that's failing due to warnings about unused variables. This PR is fine to merge with those because it's merging into feature/defer-execution-networking and those warnings are fixed in #217 which also merges into feature/defer-execution-networking. So once this PR is merged I'll rebase the branch for #217.

As mentioned before re. the failing tests - once this PR was merged I then merged feature/defer-execution-networking into defer/partial-and-incremental-execution and the test suite passes as expected.