fix(openai): return 429 for empty upstream output

[CJackHwang]

Motivation

• Upstream empty responses (not content-filtered) are semantically throttling-like; map those cases to HTTP 429 Too Many Requests instead of 502 Bad Gateway while keeping the existing error code for clients.

Description

• Change empty-output handling in internal/adapter/openai/upstream_empty.go to write http.StatusTooManyRequests for non-filtered empty upstream output while preserving error.code == "upstream_empty_output".
• Update tests in internal/adapter/openai/handler_toolcall_test.go and internal/adapter/openai/responses_stream_test.go to expect 429 for the empty-upstream non-stream paths.

Testing

• Ran the targeted unit tests with go test ./internal/adapter/openai -run "UpstreamOutputEmpty|ContentFilterErrorWhenUpstreamFilteredWithoutOutput" and they passed (ok).

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Codex Task

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Project	Deployment	Actions	Updated (UTC)
ds2api	Ready	Preview, Comment	Apr 6, 2026 8:56am