Add Access Level Inheritance

[Matejkob]

This PR introduces access level inheritance for generated spies.

Key Changes

Generated spies now inherit the protocol’s access control (public, package, internal, private, and fileprivate). For private protocols, spies are generated with fileprivate access to meet Swift’s requirements.

Spies include an initializer with the same access level as the protocol, enabling their creation outside the current module.

Implementation Details

The core concept of access level inheritance is implemented using a syntax rewriter:

final class AccessLevelModifierRewriter: SyntaxRewriter {
  let newAccessLevel: DeclModifierSyntax

  init(newAccessLevel: DeclModifierSyntax) {
    // Ensure private protocols result in fileprivate members for compatibility
    if newAccessLevel.name.text == TokenSyntax.keyword(.private).text {
      self.newAccessLevel = DeclModifierSyntax(name: .keyword(.fileprivate))
    } else {
      self.newAccessLevel = newAccessLevel
    }
  }

  override func visit(_ node: DeclModifierListSyntax) -> DeclModifierListSyntax {
    if node.parent?.is(FunctionParameterSyntax.self) == true {
      return node
    }
    return DeclModifierListSyntax { newAccessLevel }
  }
}

The AccessLevelModifierRewriter is applied after the spy class is created, ensuring that all members inherit the access level consistently:

if let accessLevel = extractor.extractAccessLevel(from: protocolDeclaration) {
  let accessLevelModifierRewriter = AccessLevelModifierRewriter(newAccessLevel: accessLevel)

  spyClassDeclaration =
    accessLevelModifierRewriter
    .rewrite(spyClassDeclaration)
    .cast(ClassDeclSyntax.self)
}

This guarantees that the entire spy class, including all members, always reflects the protocol’s access level.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.79%. Comparing base (7d07458) to head (c53222a).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   98.73%   98.79%   +0.06%     
==========================================
  Files          20       21       +1     
  Lines         949      998      +49     
==========================================
+ Hits          937      986      +49     
  Misses         12       12              

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[armintelker]

I like it, and from what I see, this is what the community wishes for. However, I can't give you a proper review because I am not deep enough into this library to understand the full impact of this PR.