fix(transport): ignore non-object postMessage data

[omerbek]

Summary

• Ignore non-object message payloads before destructuring event.data in windowPostMessageTransport
• Add a regression test for unrelated postMessage events with null and primitive payloads

Fixes #108.

Testing

• ./node_modules/.bin/vitest.cmd run src/transports/windowPostMessageTransport.test.ts
• ./node_modules/.bin/biome.cmd check src/transports/windowPostMessageTransport.ts src/transports/windowPostMessageTransport.test.ts

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Note

Low Risk
Narrow defensive guard on message filtering with a targeted unit test; no changes to request/response or auth flows.

Overview
Fixes crashes when unrelated window.postMessage traffic uses null or primitive event.data values. The listener in windowPostMessageTransport now returns early unless event.data is a non-null object, before destructuring target and data.

A regression test asserts that null and string payloads are ignored without throwing, matching behavior for wrong origin, target, or stream name.

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