Add kw_only to VWSCredentials test dataclass

[adamtheturtle]

Adds kw_only=True to the test VWSCredentials dataclass. The session fixture already passes email_address= and password=.

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Note

Low Risk
Low risk: test-only change that only affects how VWSCredentials is instantiated, with no production logic or data handling impact.

Overview
Updates the test VWSCredentials dataclass to require keyword-only initialization by adding kw_only=True, preventing accidental positional argument use in tests.

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