fix: handle undefined MessageEvent in postMessageStream

[tuna1207]

Jira Link

Description

• Guard MessageEvent.prototype access in PostMessageStream so importing @web3auth/auth does not throw in non-browser runtimes (e.g. Hermes/React Native) where MessageEvent is undefined.
• The package root re-exports PostMessageStream via the jrpc barrel, so this was an import-time failure even when consumers never instantiate the class.

How has this been tested?

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• My code follows the code style of this project. (run lint)
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

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Note

Low Risk
Small defensive guard at module init with unchanged browser behavior when MessageEvent is present; limited to JRPC postMessage stream setup.

Overview
Fixes a module-load crash when MessageEvent is missing (e.g. Hermes/React Native). postMessageStream no longer reads MessageEvent.prototype at import time; it only resolves the source/origin property getters when MessageEvent exists, and the existing _onMessageHandler fallbacks to event.origin / event.source still apply when those getters are absent.

Adds a test that deletes global MessageEvent and asserts import("../src/jrpc") still exposes PostMessageStream.

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