Consider raising an implementation diagnostic for mismatched `CustomType` / `Attributes` combinations

### Module version

```
github.com/hashicorp/terraform-plugin-framework v1.12.0
```

### Relevant provider source code

```go
func (r *thingResource) Schema(ctx context.Context, req resource.SchemaRequest, resp *resource.SchemaResponse) {
resp.Schema = schema.Schema{
Attributes: map[string]schema.Attribute{
"nested_attr": schema.SingleNestedAttribute{
// If this CustomType doesn't fully match the attributes below, you'll receive a confusing decoder error that suggests
// there is a bug in terraform-plugin-framework. This is actually a provider implementation
// error and should be surfaced earlier.
CustomType: xtypes.CustomObjectType{
ObjectType: basetypes.ObjectType{
AttrTypes: map[string]attr.Type{
"attr_1": types.StringType,

// The following line is commented out to simulate a mismatch between the custom type
// and attributes below.
// "attr_2": types.StringType,
},
},
},
Required: true,
Attributes: map[string]schema.Attribute{
"attr_1": schema.StringAttribute{
Required: true,
},
"attr_2": schema.StringAttribute{
Required: true,
},
},
},
},
}
}
```

### Terraform Configuration Files

```hcl
resource "examplecloud_thing" "this" {
nested_attr = {
attr_1 = "hello"
attr_2 = "world"
}
}
```
### Actual Behavior
When a `CustomType` for a nested attribute or nested block does not match the `Attributes`/`Blocks` defined underneath, you'll receive an error when decoding any objects from Terraform (most likely, the configuration during validation). This occurs because the provider has an invalid implementation, due to the `CustomType` attributes not matching the actual schema attributes. Framework [uses the attributes directly](https://github.com/hashicorp/terraform-plugin-framework/blob/b372415f8e8aec4de386eebba11d954d2e2c6fda/internal/toproto6/schema_attribute.go#L67-L75) for `GetProviderSchema` (which is sent to Terraform core), but for all decoding operations, [uses the actual type of the schema](https://github.com/hashicorp/terraform-plugin-framework/blob/b372415f8e8aec4de386eebba11d954d2e2c6fda/internal/fromproto5/dynamic_value.go#L35) (including the custom types).

This issue exists for all nested attributes and nested blocks that define a custom object type. You'll receive a confusing decoding error that suggests there is a bug in `terraform-plugin-framework`.

```bash
$ terraform validate
╷
│ Error: Unable to Convert Configuration
│
│ with examplecloud_thing.this,
│ on resource.tf line 13, in resource "examplecloud_thing" "this":
│ 13: resource "examplecloud_thing" "this" {
│
│ An unexpected error was encountered when converting the configuration from the protocol type. This is always an issue in terraform-plugin-framework used to implement the provider and should be reported to the provider developers.
│
│ Please report this to the provider developer:
│
│ Unable to unmarshal DynamicValue: AttributeName("nested_attr"): error decoding object; expected 1 attributes, got 2
```

### Expected Behavior

As this is a provider implementation problem, we should have received a more explicit error message indicating that the `CustomType` field does not match the `Attributes` provided and that the provider needs to update it's implementation to match the two. This can be returned during `GetProviderSchema`, similar to the other [invalid implementation diagnostics we do today](https://github.com/hashicorp/terraform-plugin-framework/blob/b372415f8e8aec4de386eebba11d954d2e2c6fda/resource/schema/set_nested_attribute.go#L277).

### References

- Semi-related to #https://github.com/hashicorp/terraform-plugin-framework/issues/947
- Semi-related to #https://github.com/hashicorp/terraform-plugin-framework/issues/774