Split a comma-joined host list into a sequence for asyncpg

[afonsojanu]

Description

A postgres DB URL with more than one host, e.g. postgres://user:pass@host1,host2:5432/db, is libpq's own multi-host syntax for primary/replica failover. urlparse reads the whole comma-joined string back as a single hostname, so asyncpg.connect() ended up being asked to resolve host1,host2 as one DNS name and failed with a plain socket.gaierror instead of trying each host in turn.

expand_db_url now splits the hostname into a list when it contains a comma and the backend is one of postgres/postgresql/asyncpg, since that's the form asyncpg's own host parameter accepts for exactly this case (it documents accepting "a sequence" of hosts, tried in order). psycopg is left untouched: it forwards its kwargs straight to libpq, which already parses the joined string correctly on its own, so splitting it there would just make libpq see a Python list where it expects the original string.

Motivation and Context

Fixes #1924

How Has This Been Tested?

Added test_postgres_multi_host to tests/backends/test_db_url.py, covering all four postgres-family schemes (postgres, postgresql, asyncpg, psycopg) against the same multi-host URL, asserting the asyncpg-family schemes get a list and psycopg keeps the joined string. Confirmed via git stash that it fails against the unmodified code (asserts a list where the code still returns the joined string) and passes with the fix.

• python -m pytest tests/backends/test_db_url.py tests/test_connection.py: 59 passed.
• ruff check and ruff format --check: clean.
• mypy on the changed file: 0 errors.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.