fix(openai): ignore empty websocket frames

[pandego]

Summary

• ignore whitespace-only websocket frames before JSON decoding
• keep streaming until a real event arrives instead of surfacing unexpected end of JSON input
• add a websocket regression test covering an empty frame before a normal completion

Testing

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

Closes #2300

[rumpl]

@pandego please fix the failing lint