Prioritize explicit endpoint options over PGHOST-derived defaults

[davoclavo]

Hello! First of all, thank you for this amazing library

This PR addresses a bug where setting the PGHOST environment variable can unintentionally override explicitly passed connection options like :hostname, :socket_dir, :socket, or :endpoints.

Summary

• Fix: Updated default_opts/1 to derive the host from PGHOST only if no endpoint-related options are explicitly provided (:socket, :socket_dir, :hostname, or :endpoints).
• Test: Added test coverage to ensure that explicitly provided connection options correctly take precedence over environment-derived defaults.
• Important note: This might cause breaking changes to existing users if they might be inadvertently overriding values that are currently ignored.

Motivation

When PGHOST is set (e.g., to a Unix socket path), Postgrex currently prioritizes it over explicitly passed options like :hostname. This causes surprising behavior: users trying to connect to a remote host (e.g., localhost over TCP) may find Postgrex silently attempting to connect over a non-existent socket.

This PR brings Postgrex closer in behavior to psql, which properly prioritizes CLI-supplied options over PGHOST.

---

Replication Instructions

You can reproduce the issue as follows:

1. Set PGHOST to any path:

   PGHOST=/whatever iex -S mix

2. Attempt to start a connection using valid TCP settings:

   Expected: Should connect over TCP to localhost
   Actual: Fails, as Postgrex attempts to connect via /whatever/.s.PGSQL.5432 socket specified via PGHOST.

   {:ok, pid} = Postgrex.start_link(
     hostname: "localhost",
     username: "postgres",
     password: "postgres",
     database: "postgres",
     port: 5432
   )
   
   Postgrex.query(pid, "SELECT 1", [])

3. Current workaround (demonstrating expected behavior with a workaround using manual override to socket_dir: nil):

   {:ok, pid} = Postgrex.start_link(
     hostname: "localhost",
     username: "postgres",
     password: "postgres",
     database: "postgres",
     port: 5432,
     socket_dir: nil
   )
   
   Postgrex.query(pid, "SELECT 1", [])

Behavior Comparison with psql

To reinforce expected behavior, here’s how psql treats similar inputs:

Explicit -h localhost takes precedence over PGHOST:

PGHOST=/whatever psql -d postgres -h localhost -U postgres -c "SELECT 1"

This should fail as no -h is provided and the PGHOST socket doesn't exist

PGHOST=/whatever psql -d postgres -U postgres -c "SELECT 1"

[josevalim]

💚 💙 💜 💛 ❤️