fix: decode SSE events without line-length limit and surface transport errors

[jiachengxu]

This PR fixes two silent-truncation defects in DoStreamingRequest's SSE decoding path:

1. Events larger than 64KiB were silently dropped. The alevinval/sse decoder reads
   lines with a default bufio.Scanner, which caps a line at bufio.MaxScanTokenSize
   (64KiB). Each streamed message arrives as a single data: {json} line, so any message
   whose JSON exceeds ~64KiB made Scan() fail with bufio.ErrTooLong.
2. Every scanner failure was flattened into io.EOF. The decoder's read loop never
   checks scanner.Err(); on any failure — the 64KiB overflow above, or a transport error
   (connection reset) mid-stream — it returns io.EOF, which DoStreamingRequest treats
   as successful completion (close(resCh)).

Combined effect: a stream carrying one oversized message, or a stream cut mid-flight,
ends exit-0 with partial data. The caller cannot distinguish it from a complete,
successful stream. #14 fixed the case where the server terminates the stream with an
{"error": ...} event; these two failures happen below that layer, on the client's own
read path, so no error event is ever seen.

How

Replace the alevinval/sse decoder with a small internal bufio.Reader-based SSE event
decoder (sseEventDecoder in pkg/grpc/gateway/request.go):

• No line-length limit — ReadString('\n') grows as needed.
• Errors propagate — read errors surface on errCh instead of closing resCh.
• Truncation is never success — an EOF that interrupts a partially-read event returns
  io.ErrUnexpectedEOF; only an EOF at a clean event boundary is a normal end-of-stream.
• SSE framing behavior is preserved: multiple data: lines of one event join with \n,
  a single leading space after the colon is trimmed, comment/heartbeat lines (: ...)
  and non-data fields are ignored, and both \n and \r\n line endings are accepted
  (the alevinval decoder handled these the same way).

Scope: only the SSE decode path used by server-streaming RPC responses. DoRequest
(unary), doHTTPStreamingRequest (google.api.HttpBody streams, which use io.Copy),
and wrapStreamingResponseError are untouched. alevinval/sse remains a test-only
dependency (marshaller_test.go uses it to decode the marshaller's output).

Testing

New regression tests in pkg/grpc/gateway/request_test.go:

Test	Covers
TestDoStreamingRequest_LargeEvents	3 × 300KiB messages through the real gRPC → grpc-gateway → SSE pipeline; each must arrive intact
TestDoStreamingRequest_TruncatedStreamSurfacesError	connection cut mid-event: the already-complete first event is delivered, then an error (never a clean close)
TestDoStreamingRequest_SSEFraming	comment/heartbeat lines, CRLF endings, one event split across multiple data: lines

All three fail against the previous decoder and pass with the fix:

[yhxlele]

LGTM