Fix: Prevent unhandled exceptions when processing empty SSE event payloads

[vazarkevych]

Fix: Empty SSE Event Payload Handling

Overview

This PR prevents unhandled exceptions when SSE events contain an empty or heartbeat payload.

Some SSE servers may send keepalive, heartbeat, ping, or empty data events. Previously, those payloads could still reach feature JSON parsing and fail when the payload was blank or invalid.

What Changed

• Added SSE payload validation before feature payload processing.
• Ignored empty and whitespace-only SSE payloads.
• Ignored heartbeat-style SSE events: heartbeat, keepalive, and ping.
• Caught parsing/runtime errors inside the async SSE listener so they do not escape as unhandled exceptions.
• Updated native Java SSE handling to skip whitespace-only payloads.
• Added tests for empty payloads, valid payloads, invalid payload handling, and heartbeat events.

[madhuchavva]

Native SSE validation path issue found.

[madhuchavva]

Requesting changes for the native SSE validation gap noted inline.