refactor: migrate external_wiki to framework

Relates to issue #6712 (closed)

Changes

This MR completes the migration of gitlab_integration_external_wiki resource from Terraform SDK to Terraform Plugin Framework.

New Framework Implementation

Created Files:

• internal/provider/resource_gitlab_project_integration_external_wiki.go - Framework-based resource implementation
• internal/provider/resource_gitlab_project_integration_external_wiki_test.go - Comprehensive test suite

Removed Files:

• internal/provider/sdk/resource_gitlab_project_integration_external_wiki.go - Legacy SDK implementation
• internal/provider/sdk/resource_gitlab_project_integration_external_wiki_test.go - Legacy SDK tests

Key Features

Dual Resource Names:

• gitlab_project_integration_external_wiki (new, recommended)
• gitlab_integration_external_wiki (deprecated, will be removed in v19.0)

Resource Attributes:

• id (Computed): Project ID
• project (Required, ForceNew): Project ID or path
• external_wiki_url (Required): External wiki URL with HTTP/HTTPS validation
• title (Computed): Integration title
• created_at (Computed): ISO8601 timestamp
• updated_at (Computed): ISO8601 timestamp
• slug (Computed): URL-safe integration name
• active (Computed): Integration status

Test Coverage:

• Basic CRUD operations with import verification
• Deprecated resource name compatibility test
• Migration test from SDK (v18.8.1) to Framework
• Destroy verification

Implementation Details

• All CRUD operations follow Framework patterns
• Proper error handling with 404 detection
• Import functionality via ImportStatePassthroughID
• Helper method modelToStateModel() for API response mapping
• Deprecation message for old resource name

API Methods Used

• client.Services.SetExternalWikiService() - Create/Update
• client.Services.GetExternalWikiService() - Read
• client.Services.DeleteExternalWikiService() - Delete