fix: replace pgxconn with pgxpool to fix pgvector connection management issue

[rickif]

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This PR replaces the usage of pgconn with pgxpool in the pgvector integration to address the connection issue caused by failed to deallocate cached statement(s): conn closed errors in pgx.

Context cancellation may close the pgxconn, causing subsequent queries to fail. More detail in jackc/pgx#2100.
pgxpool will automatically close the disconnected pgxconn and create a new one when necessary, resolving this issue.