Add Unix socket connection support to PostgreSQL session storage

[hashem59]

WHY are these changes introduced?

Fixes #3113

The PostgreSQL session storage package currently cannot connect via Unix domain sockets because postgres-connection.ts manually decomposes the connection URL into individual host, port, user, password, and database fields — dropping all query parameters in the process. This means the standard PostgreSQL ?host=/cloudsql/... query parameter (used for Unix socket paths) is silently ignored.

This is a blocker for users deploying on Google Cloud (Cloud Run, App Engine, GKE) where the Cloud SQL Auth Proxy provides connections exclusively through Unix sockets at paths like /cloudsql/project:region:instance.

This is also the root cause behind #2835 and #493 (SSL parameters dropped), and has led multiple users to fork the package to work around the limitation.

WHAT is this pull request doing?

Uses pg-connection-string — which is already a declared dependency in package.json but was never used in the connection code — to parse connection URLs instead of manual URL decomposition. This is a minimal change that:

• Properly handles Unix socket paths via the host query parameter
• Preserves SSL and all other standard PostgreSQL connection parameters (also fixes @shopify/shopify-app-session-storage-postgresql: Support for SSL Certificate Configuration in PostgreSQL Session Storage. #2835 and @shopify/shopify-app-session-storage-postgresql : error: no pg_hba.conf entry for host #493)
• Maintains full backward compatibility for existing TCP connection strings
• Adds documentation for Unix socket / Cloud SQL usage in the README
• Includes unit tests for all URL formats (TCP, Unix socket, special characters, SSL)

Verified locally against real PostgreSQL 15 with:

• TCP connection with port (postgres://user@localhost:5432/db) ✅
• TCP connection without port (postgres://user@localhost/db) ✅
• Unix socket via ?host= parameter (postgres://user@/db?host=/tmp) ✅
• Special characters in credentials and database names ✅

Usage example:

const shopify = shopifyApp({
  sessionStorage: new PostgreSQLSessionStorage(
    'postgres://username:password@/database?host=/cloudsql/my-project:us-central1:my-instance',
  ),
});

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have used pnpm changeset to create a draft changelog entry (do NOT update the CHANGELOG.md files manually)
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (for public APIs)