direct_tls doesn't work with PostgreSQL 17+ because ALPN isn't set

首先定位 asyncpg 为 direct_tls 创建的 SSL 上下文，以及处理直接 SSL 协商的连接路径。使用 issue 中的 PostgreSQL 18 Docker 命令复现，然后验证 direct_tls 连接可与默认上下文正常工作，并为 PostgreSQL ALPN 协议添加或更新覆盖率。

• asyncpg version: 0.31.0 (also checked master)
• PostgreSQL version: 17.11 and 18.0
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: No, reproduced with a local PostgreSQL in Docker
• Python version: 3.12
• Platform: Linux
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: Yes (via uv)
• If you built asyncpg locally, which version of Cython did you use?: N/A
• Can the issue be reproduced under both asyncio and
  uvloop?: Only tried asyncio

When connecting with direct_tls=True, the connection gets dropped right away:

await asyncpg.connect(dsn, ssl="require", direct_tls=True)
# ConnectionDoesNotExistError: connection was closed in the middle of operation

The server log says why:

LOG:  received direct SSL connection request without ALPN protocol negotiation extension

PostgreSQL 17 requires the postgresql ALPN protocol for direct SSL connections, and libpq always sends it. asyncpg doesn't set ALPN on the context it builds, so as far as I can tell direct_tls (and sslnegotiation=direct in the DSN) can't work against a stock PG 17+ server unless you pass your own context:

ctx = ssl.create_default_context()
ctx.set_alpn_protocols(["postgresql"])
await asyncpg.connect(dsn, ssl=ctx, direct_tls=True)  # works

Repro with Docker:

docker run --rm -e POSTGRES_PASSWORD=postgres -p 5432:5432 postgres:18 \
  -c ssl=on \
  -c ssl_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem \
  -c ssl_key_file=/etc/ssl/private/ssl-cert-snakeoil.key

I'll send a PR that sets postgresql ALPN on the contexts asyncpg creates.