fix: add validation to key_prefix in autolink reference resource (#3176)

GitHub's API enforces that key_prefix:
- must only contain letters, numbers, or .-_+=:/#
- must not end with a number

Without client-side validation, terraform plan succeeds with an invalid
key_prefix (e.g. 'PTFY25') but terraform apply fails with a 422 from
the GitHub API, giving a confusing plan/apply inconsistency.

Added ValidateDiagFunc using a regexp that mirrors the API constraints,
so invalid key_prefix values are caught at plan time.

Author: LeC-D

github/resource_github_repository_autolink_reference.go

@@ -69,6 +69,10 @@ func resourceGithubRepositoryAutolinkReference() *schema.Resource {
 				Required:    true,
 				ForceNew:    true,
 				Description: "This prefix appended by a number will generate a link any time it is found in an issue, pull request, or commit",
+				ValidateDiagFunc: validation.ToDiagFunc(validation.StringMatch(
+					regexp.MustCompile(`^[a-zA-Z0-9.=+:/#_-]*[a-zA-Z.=+:/#_-]$`),
+					"must only contain letters, numbers, or .-_+=:/# and must not end with a number",
+				)),
 			},
 			"target_url_template": {
 				Type:             schema.TypeString,