Put the default application_name into URI connection strings instead of in front of them

[PeterVanHolland]

Since #617, ATTACH 'postgresql:///mydb' AS pg (TYPE postgres) fails with a missing "=" error. This affects both postgresql:// and postgres:// URIs without application_name, including DuckLake catalogs using these paths.

The default name is prepended as application_name=DuckDB . libpq only recognizes a URI when the connection string starts with a URI prefix. It therefore reads the result as key/value pairs and rejects the URI.

This change appends the default name as a URI parameter, using the same helper as the secret's URI option. It handles empty queries, trailing separators and ? in passwords or parameter values. Tests check the default and supplied names in pg_stat_activity, with connection errors treated as failures instead of skips. They fail before the change and pass after it. The password case was checked manually over a local socket, and the other tests pass locally against Postgres.

Related: pdet/ducklake_internal#200