fix: Breaking change in MachineAuthProvider constructor

[kgabryje]

SUMMARY

A change introduced in #25247 in MachineAuthProvider caused some custom auth providers to be broken, because of unexpected second argument passed to the constructor. This PR reverts that behaviour and .allows only passing either Selenium or Playwright override auth function, not both at the same time.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

1. Add a custom MACHINE_AUTH_PROVIDER_CLASS that inherits from MachineAuthProvider and uses an __init__ method that accepts only 1 argument - custom auth function (existing implementations follow this pattern)
2. Verify that your custom auth provider works as expected.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[villebro]

I propose we keep the original design, but update the sig to conform with how this is actually used, which assumes that the overrides are actually Optional arguments that default to None.

[villebro]

LGTM, but hoping we can clean this up properly in the next major version to have the same number of overrides as auth methods.

[craig-rueda]

LGTM, but would be great if there was a test or two for this :D

[john-bodley]

@kgabryje would you mind adding your testing instructions?

[kgabryje]

Thanks for reviews!
@john-bodley Added test instructions. I had skipped this step as this PR is basically a revert to previous behaviour of the auth provider in order not to introduce a breaking change with the Playwright PR