Fix Pydantic v2 adapter_id and merged_adapters validation

[claudioMontanari]

What does this PR do?

Fixes bug introduced by #177

• Changes the way validation is performed on adapter_id and merged_adapters using pydantic model_validators
• Adds tests that cover this

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Was this discussed/approved via a Github issue or the discord / slack channel? Please add a link
  to it if that's the case.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

[tgaddair]

Thanks for following up with this fix!