fix: surface streaming errors faithfully (mid-stream on errCh + pre-first-message status)

[yhxlele]

Two related fixes so streaming errors surface faithfully instead of being lost.
Both are in DoStreamingRequest's error handling; neither requires a go.mod
change (codes/status come from the already-required google.golang.org/grpc),
and the full suite passes under the repo's CI Go 1.20 toolchain.

1. Mid-stream errors are swallowed (returned as silent success)

DoStreamingRequest's decode loop only looks for the result key:

rawResult, ok := res[streamingResponseResultKey] // "result"
if !ok { continue }

grpc-gateway frames a server-streaming RPC that fails after emitting one or
more results as a terminal data: {"error": <google.rpc.Status>} event. That
event has no result key, so the loop continues past it, the next Decode()
returns io.EOF, and the goroutine close(resCh)s — so the caller sees a clean,
successful end-of-stream and silently receives a truncated result. The
streamingResponseErrorKey constant was defined but never consulted in the loop.

Fix: check for the error key before the result key; if present, unmarshal
the google.rpc.Status and deliver it on errCh. resCh is intentionally left
un-closed on this path so a consumer's select deterministically observes the
error rather than an EOF.

2. Pre-first-message errors collapse to codes.Unknown

When a streaming handler fails before its first message, grpc-gateway sets the
HTTP error status and writes the error through the SSE marshaller, so the body is
framed as data: {"error": ...}. wrapStreamingResponseError called
json.Unmarshal on the raw body, which fails on the data: prefix and returns a
generic error — so the real gRPC status (NotFound, PermissionDenied, …)
collapses to codes.Unknown.

Fix: strip the SSE data: framing before parsing. This is a no-op for
non-streaming (plain JSON) error bodies such as routing 404s, which start with
{ rather than data:.

Tests

• TestDoStreamingRequest_ErrorAfterResults — result, result, error: asserts both
  results are delivered and then the Internal status arrives on errCh (not a
  clean EOF).
• TestDoStreamingRequest_ErrorBeforeResults — SSE-framed pre-first-message error:
  asserts the returned error carries the real NotFound status, not Unknown.

Both were verified RED before their respective fix and GREEN after. The test
service's TrackInvitation gained two id-keyed hooks (fail-after-events,
fail-before-events); the existing streaming test is unchanged.

Compatibility

errCh was already the loop's channel for decode/unmarshal errors, so callers
that drain it (the canonical select { case <-resCh; case <-errCh }) simply now
receive the real error instead of a truncated success. Fix (2) only changes the
initial error for SSE-framed bodies (Unknown → real code); it is a no-op for the
plain-JSON routing-404 case. No new contract for callers.