fix(parsers): use_proxy decorator to not override default args

[consideRatio]

Description

When WEBPROXY_USERNAME / WEBPROXY_PASSWORD is configured and a use_proxy decorated parser function was called without args/kwargs, like done when using python -m parsers.KPX instead of poetry run test-parsers KR, then the following error showed up:

Error(s) creating production breakdown Event 2025-01-06 07:30:00+09:00: 1 validation error for ProductionBreakdown
zoneKey
  none is not an allowed value (type=type_error.none.not_allowed)

It was because the use_proxy logic always passed a zone_key args explicitly, even if it wasn't received, which made a None value get passed explicitly and short-circuiting the fetch_... functions default value.

Changes

• Makes calling fetch_production() without args or kwargs not run into issues stemming from zone_key being explicitly passed as None.

• Adds a comment on what webshare.io subscription is useful for use by this project.

• Re-configures session.proxies back to what it was in a way that I think now will work, while previously I think we just stored a reference to the proxies object that we then also modified.

  This likely won't impact anyone.

• Adjusted logging level from Error to Warning. As it kept running without further complaints it felt more like a warning, and it could reasonably succeed without a proxy depending on where the code is running.

• Adjusted the code for readability, which is an opinionated matter of course.

Double check

• I have tested my parser changes locally with poetry run test_parser "zone_key"
  Things still work, with and without having WEBSHARE_... environment configured.
• I have run pnpx prettier@2 --write . and poetry run format in the top level directory to format my changes.

[consideRatio]

I'm not sure what the test failure is about, I saw it also in #7687.

Reported via #7689

[consideRatio]

Force pushed a rebase on master, hoping to see unrelated test failure pass now (see #7689).