refactor(open-sse): DRY SSE primitives into utils/sseConstants.js

Gom SSE_DONE + SSE_HEADERS + SSE_HEADERS_NO_BUFFER vào 1 file không
phụ thuộc (tránh kéo usageDb vào executor). Áp cho kiro, cursor, qoder,
github, commandcode, perplexity-web, grok-web. Byte-for-byte verified
(headers/DONE giữ nguyên; biến thể no-buffer dùng const riêng).

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/utils/stream.js

@@ -6,7 +6,10 @@ import { parseSSELine, hasValuableContent, fixInvalidId, formatSSE } from "./str
 import { getOpenAIResponsesEventName, isOpenAIResponsesTerminalEvent, formatIncompleteOpenAIResponsesStreamFailure } from "./responsesStreamHelpers.js";
 import { dbg, isDebugEnabled } from "./debugLog.js";
 
+import { SSE_DONE, SSE_HEADERS, SSE_HEADERS_NO_BUFFER } from "./sseConstants.js";
+
 export { COLORS, formatSSE };
+export { SSE_DONE, SSE_HEADERS, SSE_HEADERS_NO_BUFFER };
 
 // sharedEncoder is stateless — safe to share across streams
 const sharedEncoder = new TextEncoder();