plan: also validate provider requirements from state

[liamcervante]

This PR updates the validation of required providers so that it also makes sure any providers required by the state are included. This improves the error message returned when a resource is in state but not in config and has no included provider.

Before

╷
│ Error: failed to read schema for null_resource.resource in registry.terraform.io/hashicorp/null: failed to instantiate provider "registry.terraform.io/hashicorp/null" to obtain schema: unavailable provider "registry.terraform.io/hashicorp/null"
│ 
│ 
╵

After

╷
│ Error: Missing required provider
│ 
│ This state requires provider registry.terraform.io/hashicorp/null, but that provider isn't available. You may be able to install it automatically by
│ running:
│   terraform init
╵

Fixes #35860

Target Release

1.9.x

Draft CHANGELOG entry

ENHANCEMENTS

• Improve error message when provider is required by state but has not been downloaded.

[DanielMSchmidt]

You may be able to install it automatically

What is that dependent on? The provider being in the required provider section?

[liamcervante]

Defined anywhere in the configuration or the state. This basically catches someone running plan or apply before they ran init. We do the same check with a similar error message in case something is in config and hasn't been caught: https://github.com/hashicorp/terraform/blob/main/internal/terraform/context.go#L389

init won't install automatically if the configuration is wrong, or it's not a default provider and isn't in the required providers. But all cases init will then provide specific error messages for why something couldn't be installed.

[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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