fix mock provider validation

[ollevche]

Resolves #2139
Resolves #2038

In the initial implementation of mock providers, I missed that the validation of the provider block must also be skipped. In this PR I disable ValidateProviderConfig calls and remove provider-related schemas from GetProviderSchema call for any mocked provider.

Target Release

1.9.0

Checklist

• I have read the contribution guide.
• I have not used an AI coding assistant to create this PR.
• I have written all code in this PR myself OR I have marked all code I have not written myself (including modified code, e.g. copied from other places and then modified) with a comment indicating where it came from.
• I (and other contributors to this PR) have not looked at the Terraform source code while implementing this PR.

Go checklist

• I have run golangci-lint on my change and receive no errors relevant to my code.
• I have run existing tests to ensure my code doesn't break anything.
• I have added tests for all relevant use cases of my code, and those tests are passing.
• I have only exported functions, variables and structs that should be used from other packages.
• I have added meaningful comments to all exported functions, variables, and structs.

[github-actions]

Reminder for the PR assignee: If this is a user-visible change, please update the changelog as part of the PR.

[ollevche]

The fix should also be backported to OpenTofu 1.8