test(cooperative-api): integration tests + mock services + TESTING.md for Alyssa - #192

Starting April 2026, the CodeQL Action will skip computing file coverage information on pull requests to improve analysis performance. File coverage information will still be computed on non-PR analyses. To opt out of this change, switch to an advanced setup workflow and set the `CODEQL_ACTION_FILE_COVERAGE_ON_PRS` environment variable to `true`.