fix(mock): clear held storage instances when resetting

[OskarEichler]

Summary

Clear existing mock stores before resetting the registry and preserve the default legacy instance's registry identity.

Compatibility / observable behavior

clearAllMockStorages now actually clears previously held instances. Existing named references are cleared, while a subsequent named factory call still creates a fresh instance as before. The default legacy instance stays registered.

Related to #1331. This is a focused part of a source review; it does not claim the entire repository is bug-free.

Test Plan

• Actual compiled shipped mock: default and named held references both retain values after baseline reset; fixed clears both and keeps default === createAsyncStorage('legacy'). Public signatures remain unchanged.
• Upstream yarn build:js, yarn test:ts (6 tasks), yarn test:lint (6 tasks), yarn test:format and git diff --check pass on the combined review checkout.
• No checked-in test/spec files changed. Reproductions were isolated and used synthetic data, not production/user storage.
• The combined 3.1.1 backport passes consumer immutable install/lint, both products' Android Debug and iOS Simulator Debug builds, four production Metro bundles, 13 web targets and four browser-extension targets. Published native framework binaries are unchanged. React Doctor: 100/100.
• Shared-storage testAndroidHostTest, macosArm64Test and iosSimulatorArm64Test pass with Java 21. The first Android run used Java 17 and failed Robolectric's SDK 36/JDK check; rerunning with the project's documented JDK fixed the environment failure without a source workaround.
• Windows/WinRT runtime, signed releases and physical-device behavior have not been verified locally.

[changeset-bot]

🦋 Changeset detected

Latest commit: f4ef651

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@react-native-async-storage/async-storage	Patch

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