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

.github/actions/neon-project-create/action.yml

@@ -49,10 +49,6 @@ inputs:
     description: 'A JSON object with project settings'
     required: false
     default: '{}'
-  default_endpoint_settings:
-    description: 'A JSON object with the default endpoint settings'
-    required: false
-    default: '{}'
 
 outputs:
   dsn:
@@ -139,21 +135,6 @@ runs:
             -H "Accept: application/json" -H "Content-Type: application/json" -H "Authorization: Bearer ${ADMIN_API_KEY}" \
             -d "{\"scheduling\": \"Essential\"}"
         fi
-        # XXX
-        # This is a workaround for the default endpoint settings, which currently do not allow some settings in the public API.
-        # https://github.com/neondatabase/cloud/issues/27108
-        if [[ -n ${DEFAULT_ENDPOINT_SETTINGS} && ${DEFAULT_ENDPOINT_SETTINGS} != "{}" ]] ; then
-          PROJECT_DATA=$(curl -X GET \
-              "https://${API_HOST}/regions/${REGION_ID}/api/v1/admin/projects/${project_id}" \
-              -H "Accept: application/json" -H "Content-Type: application/json" -H "Authorization: Bearer ${ADMIN_API_KEY}" \
-              -d "{\"scheduling\": \"Essential\"}"
-          )
-          NEW_DEFAULT_ENDPOINT_SETTINGS=$(echo ${PROJECT_DATA} | jq -rc ".project.default_endpoint_settings + ${DEFAULT_ENDPOINT_SETTINGS}")
-          curl -X POST --fail \
-                "https://${API_HOST}/regions/${REGION_ID}/api/v1/admin/projects/${project_id}/default_endpoint_settings" \
-                -H "Accept: application/json" -H "Content-Type: application/json" -H "Authorization: Bearer ${ADMIN_API_KEY}" \
-                --data "${NEW_DEFAULT_ENDPOINT_SETTINGS}"
-        fi
         
 
       env:
@@ -171,4 +152,3 @@ runs:
         PSQL: ${{ inputs.psql_path }}
         LD_LIBRARY_PATH: ${{ inputs.libpq_lib_path }}
         PROJECT_SETTINGS: ${{ inputs.project_settings }}
-        DEFAULT_ENDPOINT_SETTINGS: ${{ inputs.default_endpoint_settings }}