Condition CombineSchedulers dependency on iOS

[yqiang]

Summary

Condition the CombineSchedulers target dependency on iOS.

This keeps ResponsiveTextField’s supported platforms unchanged while allowing macOS SwiftPM consumers to resolve packages that include ResponsiveTextField only as an iOS-conditional dependency.

Motivation

ResponsiveTextField is iOS-only, but its dependency on CombineSchedulers is currently unconditional.

When a macOS package graph contains ResponsiveTextField behind an iOS platform condition, SwiftPM still validates its unconditional dependencies. Resolution then fails because ResponsiveTextField implicitly requires macOS 10.13 while CombineSchedulers requires macOS 10.15:

the library 'ResponsiveTextField' requires macos 10.13, but depends on the product 'CombineSchedulers' which requires macos 10.15

Conditioning the dependency on iOS avoids this validation failure without incorrectly declaring that ResponsiveTextField supports macOS.

Verification

Created a macOS SwiftPM consumer that references ResponsiveTextField using:

.product(
    name: "ResponsiveTextField",
    package: "swift-responsive-textfield",
    condition: .when(platforms: [.iOS])
)