fix: Misc. .NET/github action warnings

[stefanedwards]

Fixes most of the compiler warnings shown in #170, except for the MongoVectorExtension.

Notes:

• This fix changes the behaviour of validation errors, as they are now required to have a non-null error message.
• The "possible null reference" in LoadTableSchema in Extensions/PostgreSQL/PostgresqlDataSinkExtension.cs lines could potentially be entirely resolved by iterating over GetColumnSchema instead of converting to a DataTable.
• The unit test Invoke_ForTestExtension_ProducesValidSettingsJson appears broken, as the output of the mock-settings are null.
• Added "-graphBuild:true" to validate.yml workflow, as it appears to help issues mentioned in Workaround to fix VS build hang in certain scenarios #164.

[markjbrown]

@stefanedwards this is failing the build test. Can you take a look?

[philnach]

I fixed the merge and also confirmed that the tests build and run locally, I kicked of the actions to confirm that the actions still pass.