chore: signature for the $headers param in FeatureTestTrait::withHeaders()

[michalsn]

Description
This PR fixes the signature for the $headers parameter in FeatureTestTrait::withHeaders(). The type was incorrectly defined as array<string, Header|list<Header>>, but the values passed to setHeader() are raw strings (or arrays of strings), not Header objects. The correct type is array<string, string|list<string>>.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

Thanks for the reviews!