fix: add missing Codex CLI headers for OAuth account test

The testOpenAIAccountConnection function was sending requests to
chatgpt.com/backend-api/codex/responses without the required Codex CLI
identification headers, causing 401 Unauthorized for OAuth/PAT accounts.

The gateway Forward path and the compact test path both set these headers:
- Originator: codex_cli_rs
- User-Agent: codex_cli_rs/<version>
- OpenAI-Beta: responses=experimental

This patch adds the same headers to the normal (non-compact) OAuth test
path. API key accounts and other platforms are unaffected since the
headers are inside the isOAuth branch.

backend/internal/service/account_test_service.go

@@ -600,6 +600,9 @@ func (s *AccountTestService) testOpenAIAccountConnection(c *gin.Context, account
 	if isOAuth {
 		req.Host = "chatgpt.com"
 		req.Header.Set("accept", "text/event-stream")
+		req.Header.Set("OpenAI-Beta", "responses=experimental")
+		req.Header.Set("Originator", "codex_cli_rs")
+		req.Header.Set("User-Agent", codexCLIUserAgent)
 		setOpenAIChatGPTAccountHeaders(req.Header, credentialAccount)
 	}