[management] Return the reason when custom domain validation fails (#8122)

* [management] Return the reason when custom domain validation fails

The validate endpoint returned an empty 202 and ran the check in a
goroutine, so a failed verification only reached the management log.
Run the check in the request and return 204 or 412 with a classified
reason. Resolver errors are not forwarded.

* [management] Address review feedback on custom domain validation

Pass the request context to the lookup and store calls now that the
check runs in the request, compare the self-resolved name
case-insensitively, document the 412 like the other entries, and
check the returned error in the existing tests.

* [management] Match custom domain CNAME targets case-insensitively

DNS names are case-insensitive, so a CNAME pointing at the cluster in
different casing should validate instead of failing as a mismatch.

* [management] Add validation timeout

---------

Co-authored-by: DevBot NetBird <91386968+netbirddev@users.noreply.github.com>

shared/management/http/api/openapi.yml

@@ -12895,7 +12895,7 @@ paths:
   /api/reverse-proxies/domains/{domainId}/validate:
     get:
       summary: Validate a custom domain
-      description: Trigger domain ownership validation for a custom domain
+      description: Validate domain ownership for a custom domain
       tags: [ Services ]
       security:
         - BearerAuth: [ ]
@@ -12908,8 +12908,8 @@ paths:
             type: string
           description: The custom domain ID
       responses:
-        '202':
-          description: Reverse proxy custom domain validation triggered
+        '204':
+          description: Reverse proxy custom domain validated
         '400':
           "$ref": "#/components/responses/bad_request"
         '401':
@@ -12918,6 +12918,9 @@ paths:
           "$ref": "#/components/responses/forbidden"
         '404':
           "$ref": "#/components/responses/not_found"
+        '412':
+          description: Precondition failed - the custom domain could not be validated
+          content: { }
         '500':
           "$ref": "#/components/responses/internal_error"
   /api/integrations/notifications/types: