salesforce: deep dive on unit tests

Once the new salesforce API is stabilised, I want to do a deep dive on unit testing

We should use unidici mocks to mock out the requests.

I want a really clean mock API with good clean, re-usable mock data. I am tempted to bring #482 into this, although there's unrelated stuff in that PR.

I want to set a standard for unit tests on core adaptors here. We should test the happy path and different options, expand references, and error handling.

Every Operation should have a bunch of tests:

• basic functionality (ie, writes to state.data)
• expandable references
• any particularly complex options need a test
• common error patterns

I probably wouldn't bother testing references

Tests should be small, focused and precise. Each test should test exactly one thing, and do it thoroughly

Start by locating the Salesforce adaptor and its existing tests, then review the proposed relationship to PR #482 and the use of unidici mocks. Done means establishing focused unit-test coverage for operations, options, expandable references, and common errors with reusable mock data, but the issue does not name specific files or tests.