[codex] fix Responses SSE content part lifecycle for streaming fallback

[jerryan999]

Summary

• emit response.content_part.added before the first response.output_text.delta for streamed text message items
• emit response.content_part.done after response.output_text.done with the final accumulated text
• cover both chat-completions-to-responses and anthropic-to-responses bridges with regression tests

Root Cause

The Responses streaming bridges created response.output_item.added and then streamed response.output_text.delta directly, but never opened or closed the active text content part. Clients that enforce the Responses event model, including Codex CLI, therefore rejected text deltas because no active content part existed.

Validation

• go test ./internal/pkg/apicompat
• go test -tags unit ./internal/service -run 'TestForwardResponses_ForceChatCompletionsRoutesStreamingToChatCompletions$'

Fixes #2875

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[jerryan999]

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