aisix-a2a: an SSE frame whose JSON-RPC envelope spans several data: lines fails the stream

aisix-a2a's SSE relay reads one data: line as one JSON-RPC envelope, so an agent that spells a long envelope across several data: lines fails the whole stream rather than being read.

sse_events (crates/aisix-a2a/src/bridge.rs) splits the byte stream on \n and hands each line to parse_sse_data_line, which does strip_prefix("data:") and parses the remainder as a complete envelope. The event-stream spec joins a frame's data: lines with \n into one payload, so a continued envelope arrives as two invalid fragments — and the first one returns A2aError::Request("malformed JSON-RPC event"), whose arm returns and ends the stream mid-task.

The assumption is deliberate and documented in that function's doc comment ("A2A carries one JSON-RPC envelope per data: line"), which is why #1142 fixed the same reader class in aisix-proxy and left this one alone rather than changing it silently. Closing it is not a one-line edit: it moves the relay from line framing to frame framing, and with that the point at which a malformed line fails the stream — today the first bad line fails immediately, whereas a frame reader has to buffer to the blank line before it can decide. That trade is worth making deliberately.

Same class as #1105 and #1100.

Start in crates/aisix-a2a/src/bridge.rs by reading sse_events and parse_sse_data_line, including the documented one-envelope-per-data-line assumption. Compare the related aisix-proxy reader change in #1142 and review #1105 and #1100 for context. Done means an SSE frame can join multiple data: lines into one JSON-RPC envelope while preserving deliberate malformed-frame error behavior.