Improve errdetail for logical replication conflict messages.

Improve errdetail for logical replication conflict messages.

This change enhances the clarity and usefulness of error detail messages
generated during logical replication conflicts. The following improvements
have been made:

1. Eliminate redundant output: Avoid printing duplicate remote row and
replica identity values for the multiple_unique_conflicts conflict type.
2. Improve message structure: Append tuple values directly to the main
error message, separated by a colon (:), for better readability.
3. Simplify local row terminology: Remove the word 'existing' when
referring to the local row, as this is already implied by context.
4. General code refinements: Apply miscellaneous code cleanups to improve
how conflict detail messages are constructed and formatted.

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Shveta Malik <shveta.malik@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Zhijie Hou <houzj.fnst@fujitsu.com>
Discussion: https://postgr.es/m/CAHut+Psgkwy5-yGRJC15izecySGGysrbCszv_z93ess8XtCDOQ@mail.gmail.com