feat: add transform method to request context

[RYNBSD]

• Add transform method that allows functional updates to context values
• Method takes a key and transformation function for immutable updates
• Support complex data structures and object transformations
• Maintain request isolation and work with default store values
• Add TypeScript definitions for type-safe transformations
• Include comprehensive test coverage for various use cases

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[climba03003]

I see it as a helper function to reduce the call of .get and .set.

But I don't see how it can be immutable updates.

[RYNBSD]

transform is a helper that enables immutable updates when the passed fn returns a new value (object/array/primitive).

It does not itself deep-clone or prevent mutation of the old object — immutability is achieved by the user-supplied fn or by making fn operate on a copy/frozen value.

To make immutable updates safe by default, ensure:
Each request gets a cloned store (per-request isolation), and
Document/prefer fn implementations that return new objects (or optionally pass a copy/frozen value into fn).