Chore: GCP Postgres integration tests

[vchan]

No description provided.

[Copilot]

Pull Request Overview

This PR adds integration test support for GCP Postgres by configuring the necessary test infrastructure and updating connection validation logic.

• Adds GCP Postgres as a new integration test engine
• Updates test configuration to handle the gcp_postgres engine type
• Removes conflicting password/IAM authentication validation to allow both authentication methods

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/core/engine_adapter/integration/init.py	Registers GCP Postgres as a cloud-based integration test engine
tests/core/engine_adapter/integration/test_integration.py	Adds logic to properly handle the gcp_postgres engine type during test initialization
tests/core/engine_adapter/integration/config.yaml	Defines GCP Postgres connection configuration with environment variables
sqlmesh/core/config/connection.py	Removes validation that prevented using both password and IAM authentication
Makefile	Adds gcp-postgres-test target with required environment variable guards
.circleci/manage-test-db.sh	Implements GCP Postgres database management functions using Cloud SQL Proxy
.circleci/continue_config.yml	Integrates GCP Postgres into CI workflow and sets database environment variable

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