Add readonly support to constraint interfaces for better TypeScript compatibility

[Copilot]

Problem

Users were unable to pass readonly arrays and objects as constraint parameters without encountering TypeScript compilation errors. This created friction for codebases that prefer readonly data structures for better immutability guarantees.

Consider this common scenario:

// ❌ Previously caused TypeScript errors
const readonlyValues: readonly Arbitrary<unknown>[] = [fc.boolean(), fc.integer()];
fc.object({ values: readonlyValues }); // Type error!

const readonlySchemes: readonly string[] = ['http', 'https'] as const;
fc.webUrl({ validSchemes: readonlySchemes }); // Type error!

Solution

Updated constraint interfaces to accept readonly arrays while maintaining full backward compatibility:

• ObjectConstraints.values now accepts readonly Arbitrary<unknown>[]
• WebUrlConstraints.validSchemes now accepts readonly string[]
• ArrayConstraintsInternal.experimentalCustomSlices now accepts readonly (readonly T[])[]

The implementation leverages TypeScript's contravariance rules where functions accepting readonly T[] can also accept T[], ensuring existing code continues to work unchanged.

After this change

// ✅ Now works perfectly!
const readonlyValues: readonly Arbitrary<unknown>[] = [fc.boolean(), fc.integer()];
fc.object({ values: readonlyValues }); // No errors ✅

const readonlySchemes: readonly string[] = ['http', 'https'] as const;
fc.webUrl({ validSchemes: readonlySchemes }); // No errors ✅

// ✅ Existing mutable arrays still work (backward compatibility)
const mutableValues = [fc.boolean(), fc.integer()];
fc.object({ values: mutableValues }); // Still works ✅

Technical Details

• Updated internal helper functions to safely handle readonly arrays using spread operator when conversion to mutable arrays is needed
• Modified the entire constraint processing pipeline from interfaces down to implementation classes
• Added comprehensive tests demonstrating readonly constraint functionality
• Ensured no runtime behavior changes - this is purely a TypeScript typing improvement

This change makes fast-check more compatible with modern TypeScript codebases that prefer readonly data structures while maintaining complete backward compatibility.

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