Add callback request registry

[vegaro]

Motivation

This is the first part of #1008 which fixes the fact that Purchases stores one mutable callback per operation. Two overlapping calls can overwrite each other, so the first response invokes the wrong callback and the other response is dropped.

Stack

1. This PR
2. Pass Unity callback request IDs through Android #1067
3. Pass Unity callback request IDs through iOS #1068
4. Fix concurrent Unity callback routing #1008

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Note

Low Risk
New internal helper and tests only; no wiring into purchase flows yet.

Overview
Introduces an internal CallbackRegistry so async operations can stash callbacks under unique request IDs instead of sharing a single mutable slot (foundation for fixing overlapping Purchases calls in later stack PRs).

Register<T> returns a GUID string and stores the callback (including null) with its type; TryTake<T> removes and returns the callback only when the ID exists and the type matches—wrong types leave the entry intact. Clear drops all pending entries. Access is guarded with a lock.

Adds EditMode NUnit tests for out-of-order take, single-use IDs, null callbacks, type mismatch, and clear behavior, plus Unity .meta assets.

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[facumenzella]

nice work 🙌

heads up, the three red jobs (build-subtester-android, build-subtester-ios, build-integration-tests-ios) all failed on unity/prepare-env,