Fix alias_dsn detection for --list-dsn and -D

[ChrisJr404]

Description

--list-dsn and -D/--dsn read the config with a bare load_config(pgclirc, config_full_path) that skips the packaged default pgclirc, unlike everything else in pgcli which goes through get_config. That's the root cause of #1489.

Two ways it bites:

• --list-dsn runs before PGCli() is constructed, so on a fresh install the config file hasn't been written yet. load_config then reads a non-existent file, cfg["alias_dsn"] raises KeyError, and the broad except Exception swallows it and prints Invalid DSNs found in the config file with exit 1. No template gets written either, so there's nothing for the user to edit. To someone who did add an alias it just looks like the section is being ignored.
• -D <alias> has the same bare read plus a cfg["main"] access further down (timezone handling). If the user's config only contains an [alias_dsn] block, [main] isn't there and it blows up after resolving the alias.

Switching both call sites to get_config(pgclirc) fixes it: get_config writes the default template when the file is missing and merges the packaged default, so the expected sections ([main], [alias_dsn]) are always present and aliases resolve the same way they do everywhere else.

I couldn't reproduce the original report on a config that already has a populated [alias_dsn] (that path works), but the fresh/minimal-config cases above are reproducible and match the reported symptoms.

Tests in tests/test_alias_dsn.py: fresh config now lists nothing and exits 0 instead of erroring; populated [alias_dsn] is listed; -D <alias> resolves to the right connection; -D <unknown> still errors. The fresh-config and -D-resolve tests fail on main and pass with this change.

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