Add documentation and validation for Slack V2

[victorasu]

Problem:
Users were unaware of how to use the TF provider to provision Slack V2 notifications.

Solution:
Add documentation for Slack V2 and validations for the required fields to dbtcloud_notification and dbtcloud_partial_notification.

[Copilot]

Pull request overview

This PR updates the Terraform provider’s notification resources to better support Slack account-level (V2) notifications by documenting the new notification_type = 5, adding config-time validation for required fields, and improving error details when the Slack V2 integration is not enabled.

Changes:

• Document notification_type = 5 (Slack V2) across schemas, docs, and examples.
• Add ValidateConfig checks requiring slack_channel_id for Slack notification types (2 and 5).
• Improve create/update error details to provide actionable guidance when Slack V2 fails with a not-found error; add an acceptance test for validation behavior.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pkg/framework/objects/partial_notification/schema.go	Updates notification_type description to include Slack V2 (type 5).
pkg/framework/objects/partial_notification/resource.go	Adds Slack V2 required-field validation and augments error details for type 5 failures.
pkg/framework/objects/notification/schema.go	Updates notification_type description to include Slack V2 (type 5).
pkg/framework/objects/notification/resource.go	Adds Slack V2 required-field validation and augments error details for type 5 failures.
pkg/framework/objects/notification/resource_acceptance_test.go	Adds an acceptance test for config-time validation scenarios.
examples/resources/dbtcloud_notification/resource.tf	Adds an example of Slack V2 notification configuration.
docs/resources/partial_notification.md	Updates generated docs text for notification_type to include Slack V2 (type 5).
docs/resources/notification.md	Adds Slack V2 example and updates docs text for notification_type to include Slack V2 (type 5).

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