Add codegen-serde unit tests to CI (#4314)

This PR adds the codegen-serde module to the GitHub Actions test matrix
& fixes broken tests.

Along the way, I found some broken tests which I also fixed.

## Changes
- Added `check-serde-codegen-unit-tests` script in `tools/ci-scripts/`
- Added corresponding target to `ci.mk`
- Added the test to the GitHub Actions workflow matrix in
`.github/workflows/ci.yml`

This follows the same pattern as other codegen unit test modules
(codegen-core, codegen-client, codegen-server) and ensures that
codegen-serde tests are run as part of CI.