test(open-sse): add P0 golden tests (url/header, response stream, request body)

Lock current behavior before refactor: buildUrl/buildHeaders per default-executor
provider, translateResponse streaming (claude/gemini), translateRequest body
(openai->claude/gemini/kiro). Sanitize volatile fields (tokens, kimi device-id,
kiro conversationId, timestamps) for stable snapshots.

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

tests/__baseline__/current.json

@@ -0,0 +1 @@
+{"numTotalTestSuites":219,"numPassedTestSuites":202,"numFailedTestSuites":17,"numPendingTestSuites":0,"numTotalTests":823,"numPassedTests":777,"numFailedTests":26,"numPendingTests":20,"numTodoTests":0,"snapshot":{"added":0,"failure":false,"filesAdded":0,"filesRemoved":0,"filesRemovedList":[],"filesUnmatched":0,"filesUpdated":0,"matched":149,"total":149,"unchecked":0,"uncheckedKeysByFile":[],"unmatched":0,"updated":0,"didUpdate":false},"startTime":1781342494652,"success":false,"testResults":[{"aLine truncated