fix: surface completed title responses

[pandego]

Summary

• surface final message text from response.completed / response.done when the stream did not emit text deltas
• preserve the existing finish-reason handling so title generation can succeed for GPT-5 responses that only include text in the terminal event
• add websocket regression coverage for completed-response-only message output

Closes #2318

Testing

• docker run --rm -v "$PWD":/src -w /src golang:1.26 go test ./pkg/model/provider/openai -run 'TestWSStream_(TextDelta|CompletedResponseIncludesFinalMessageText|ToolCall|EndToEnd_WithClient)' -count=1