fix: preserve multiline SSE events

[Marve10s]

An SSE event with several data: fields currently triggers one callback per line. A JSON object split across three fields therefore arrives as three strings instead of one object.

Feed intact text chunks to Effect's SSE parser and parse JSON after each completed event. Heartbeat detection keeps its line-based handling on the same shared response body, and server retry: directives leave the existing reconnect policy intact.

This follows SSE framing: raw whitespace is preserved, events dispatch on a blank line, and incomplete trailing events are discarded. Effect's existing limit on pending parser state also applies.

The tests retain a passing single-line control and catch three framing regressions on main. All pass with the fix and also verify cancellation when unsubscribing.