Fix Cloud Extensions Regression (#11907)

## Problem
The regression test on extensions relied on the admin API to set the
default endpoint settings, which is not stable and requires admin
privileges. Specifically:
- The workflow was using `default_endpoint_settings` to configure
necessary PostgreSQL settings like `DateStyle`, `TimeZone`, and
`neon.allow_unstable_extensions`
- This approach was failing because the API endpoint for setting
`default_endpoint_settings` was changed (referenced in a comment as
issue #27108)
- The admin API requires special privileges.
## Summary of changes
We get rid of the admin API dependency and use ALTER DATABASE statements
instead:
**Removed the default_endpoint_settings mechanism:**
- Removed the default_endpoint_settings input parameter from the
neon-project-create action
- Removed the API call that was attempting to set these settings at the
project level
- Completely removed the default_endpoint_settings configuration from
the cloud-extensions workflow
**Added database-level settings:**
- Created a new `alter_db.sh` script that applies the same settings
directly to each test database
- Modified all extension test scripts to call this script after database
creation

docker-compose/ext-src/pg_graphql-src/regular-test.sh

@@ -18,6 +18,7 @@ TESTS=${TESTS/row_level_security/}
 TESTS=${TESTS/sqli_connection/}
 dropdb --if-exist contrib_regression
 createdb contrib_regression
+. ../alter_db.sh
 psql -v ON_ERROR_STOP=1 -f test/fixtures.sql -d contrib_regression
 ${REGRESS} --use-existing --dbname=contrib_regression --inputdir=${TESTDIR} ${TESTS}