Avoid failure to open dropped detached partition

Avoid failure to open dropped detached partition

When a partition is detached and immediately dropped, a prepared
statement could try to compute a new partition descriptor that includes
it.  This leads to this kind of error:
ERROR:  could not open relation with OID 457639

Avoid this by skipping the partition in expand_partitioned_rtentry if it
doesn't exist.

Noted by me while investigating bug #18559.  Kuntal Gosh helped to
identify the exact failure.

Backpatch to 14, where DETACH CONCURRENTLY was introduced.

Author: Álvaro Herrera <[email protected]>
Reviewed-by: Kuntal Ghosh <[email protected]>
Reviewed-by: Junwang Zhao <[email protected]>
Discussion: https://postgr.es/m/202408122233[email protected]