fix: use @model.bucket instead of CouchbaseOrm::Connection.bucket in delete_all

`delete_all` was the only method in `CouchbaseOrm_Relation` still using
the global `CouchbaseOrm::Connection.bucket`. All other storage accesses
(`to_n1ql`, `update_all`, `execute`, `first`, `last`) correctly use
`@model.bucket` or `@model.cluster`.

This inconsistency was a historical oversight: `delete_all` was
introduced in the first commit of the relation class before
`@model.bucket` existed, and was never updated when the abstraction
was added in later commits.

This fix makes `delete_all` consistent with the rest of the class and
enables per-model dynamic connection routing (e.g. Couchbase Capella).

Author: pimpin

lib/couchbase-orm/relation.rb

@@ -113,7 +113,7 @@ def [](*args)
             def delete_all
                 CouchbaseOrm::logger.debug{ "Delete all: #{self}" }
                 ids = query.to_a
-                CouchbaseOrm::Connection.bucket.default_collection.remove_multi(ids) unless ids.empty?
+                @model.bucket.default_collection.remove_multi(ids) unless ids.empty?
             end
 
             def where(string_cond=nil, **conds)