AppSync Events API WebSocket fails with angle brackets in payload

[shivennn]

WebSocket connections fail when publishing events containing angle brackets (<>)
with "BadRequestException: signature mismatch", while HTTP POST works fine.

Steps to Reproduce:

1. Create AppSync Events API with IAM auth
2. Publish via HTTP POST: {"channel":"/test","events":["{\"event_1\":\"<test>\"}"]} ✅ Works
3. Publish via WebSocket: same payload ❌ Fails with signature error

Expected: Both HTTP and WebSocket should handle angle brackets consistently
Actual: WebSocket requires Base64 encoding as workaround

Workaround: Encode angle brackets as Base64 before WebSocket publish