plugin: return missing identity schema diagnostics in ReadResource

[fallintoplace]

What changed

This updates the gRPC provider ReadResource implementations for protocol 5 and protocol 6 to return immediately after appending the missing identity schema diagnostic.

I also added matching regression tests for both implementations.

Why

If a provider returns NewIdentity for a resource type whose schema does not include an identity schema, Terraform currently appends an error diagnostic and then continues into resSchema.Identity.ImpliedType(). That can panic instead of returning the diagnostic.

Testing

• go test ./internal/plugin -run 'TestGRPCProvider_ReadResource(_missingIdentitySchema)?$'
• go test ./internal/plugin6 -run 'TestGRPCProvider_ReadResource(_missingIdentitySchema)?$'
• go test ./internal/plugin
• go test ./internal/plugin6

[hashicorp-cla-app]

All committers have signed the CLA.

[hashicorp-cla-app]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

Learn more about why HashiCorp requires a CLA and what the CLA includes

_{Have you signed the CLA already but the status is still pending? Recheck it.}