Expose the gateway factory name as QuickpayGatewayFactory::NAME

[loevgaard]

Closes #54. Upstream half of Setono/SyliusQuickpayPlugin#103.

use Setono\Payum\Quickpay\QuickpayGatewayFactory;

QuickpayGatewayFactory::NAME;   // 'quickpay'

This package sets payum.factory_name, so it is the authority on the value — but it exposed no
constant, leaving every consumer to repeat the literal. The factory now uses the constant in its own
default config, so the two cannot drift, and the README documents it where the gateway options are
described.

On the test

It asserts the literal 'quickpay' directly rather than only through the constant. The value, not
the symbol, is the contract: consumers store it as factoryName on their gateway configurations, so
changing it would orphan every one of them. Asserting the constant against itself would prove nothing
and let a rename through unnoticed.

Two things I did not do

• The class is still not final. The snippet in Expose the gateway factory name as a constant (QuickpayGatewayFactory::NAME) #54 shows final class QuickpayGatewayFactory, but
  the task list does not ask for it and it would be a real BC break — Payum gateway factories are
  routinely extended to preset options. Happy to do it separately if that was intended rather than
  incidental.
• The release-notes task (Expose the gateway factory name as a constant (QuickpayGatewayFactory::NAME) #54's second item) is left until this merges. The v2.0.0-alpha.4 draft
  targets 2.x by branch, so its notes should only claim this once it is actually in the tag. I will
  add the line then.

Verified

226 tests, PHPStan level 8, ECS. No behaviour change — the constant resolves to the same string the
factory already published.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.43%. Comparing base (a4619b7) to head (87eb2b7).

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x      #55   +/-   ##
=========================================
  Coverage     97.43%   97.43%           
  Complexity      126      126           
=========================================
  Files            14       14           
  Lines           351      351           
=========================================
  Hits            342      342           
  Misses            9        9           

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