Fixes#4653
## Problem
The /v1/responses endpoint (Responses → Anthropic bridge) failed when
upstream providers returned SSE events in compact format without spaces
after colons (e.g. 'event:message_start' instead of 'event: message_start').
This affected Kimi API (api.kimi.com) and potentially other upstreams,
causing 'Upstream stream ended without a response' errors.
## Root Cause
The SSE parser in gateway_forward_as_responses.go used strict prefix
matching with hardcoded spaces:
if !strings.HasPrefix(line, "event: ") { ... }
if !strings.HasPrefix(dataLine, "data: ") { ... }
According to the SSE spec
(https://html.spec.whatwg.org/multipage/server-sent-events.html),
the space after the colon is optional. Both formats are valid:
event: message_start ✓
event:message_start ✓
## Solution
Introduced parseAnthropicSSEField() helper that:
- Accepts both 'field: value' and 'field:value' formats
- Uses TrimSpace() to handle optional/multiple spaces
- Returns (value, ok) for consistent error handling
Applied to both streaming handlers:
- handleResponsesBufferedStreamingResponse
- handleResponsesStreamingResponse
## Testing
Added comprehensive unit tests:
- TestParseAnthropicSSEField: validates parser with 8 edge cases
- TestHandleResponsesBufferedStreamingResponse_CompactSSEFormat
- TestHandleResponsesStreamingResponse_CompactSSEFormat
All existing tests pass, confirming backward compatibility.
suuuuuu-1
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