Improve `apiComplianceSuite` callback checks

[fix]/[plan] It occurs to me, we don't test that the appropriate index (_idx) is given by the manager implementation in any of our batch method tests in apiComplianceSuite.

We should probably use Mocks instead of lambdas to properly assert calls, and extract the arguments from its call_args for further inspection.

Alternatively, we can make sure we use the idx argument for implicit testing, e.g. errors = [None] then lambda idx, error: operators.setitem(errors, idx, error). But that has the disadvantage that we can't assert that the callback was only called once.

As a side note, adding an equality operator to BatchElementError would make testing easier, especially for the likes of assert_called_once_with, assert_has_calls, etc.

Originally posted by @feltech in https://github.com/OpenAssetIO/OpenAssetIO/pull/597#discussion_r959804024

The work is in the apiComplianceSuite batch method tests; start by reviewing how callbacks are currently supplied and how BatchElementError is represented. Update the tests to inspect the callback index and verify callback call counts, then run the relevant batch method tests to confirm the expected errors and calls.