store: Populate fulltext columns when copying a subgraph

CopyEntityBatchQuery::new looked up source columns via Table::column,
which deliberately hides tsvector (fulltext) columns. Because fulltext
columns are nullable, they were silently dropped from the copy instead
of hitting the non-nullable error path, so the destination's fulltext
search column and its GIN index were left empty after a copy/graft.

Add Table::column_including_fulltext and use it in the copy so fulltext
columns are matched and their already-computed tsvector value is copied
verbatim; document that Table::column skips fulltext search columns. Add
a regression test that copies the User table and verifies a fulltext
query against the copy returns the expected entity.