Add connection to hx-sse and hx-multipart event details

[scriptogre]

PR #3910 exposes the connection on hx-ws message and error events.

This aligns hx-sse and hx-multipart:

• SSE message events include connection.
• Multipart part events include connection.
• Stream and reconnect errors include connection.
• SSE setup errors keep url because no connection exists yet.

The event's connection is the same object exposed by its before/after connection events.

[1cg]

Thank you @scriptogre!