Add 'explicit configuration required' message when provider schema validation fails

[liamcervante]

Currently, when the ValidateProviderConfig RPC fails as part of schema validation for a generated provider configuration block, we add an additional sourceless diagnostic that adds some context around the provider that failed saying the user should add an explicit configuration block.

We do not currently do this if the schema validation itself fails (such as missing a required attribute). This PR adds the same error message we generate for the failed provider validation call to the earlier schema validation. The logic is exactly the same: if a generated configuration block fails validation it means the user must write it themselves.

Fixes #34594
Fixes #28482

Target Release

1.7.2

Draft CHANGELOG entry

ENHANCEMENTS

• Add additional diagnostics when a generated provider block that fails schema validation requires explicit configuration.

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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