fix(core): fall back on oversized websocket requests

[neriousy]

Summary

• detect WebSocket close code 1009 as a transport size failure
• retry the rejected request immediately over HTTP and keep the session on HTTP
• preserve the existing retry budget for transient WebSocket closes

Fixes #42572

Reproduction

Requires ffmpeg and OpenAI authentication. From the repository root:

ffmpeg -y -loglevel error \
  -f lavfi -i "nullsrc=s=1920x1080:r=1,noise=alls=100:allf=t+u" \
  -frames:v 6 /tmp/opencode-ws-%d.png

cd packages/opencode
OPENCODE_EXPERIMENTAL_WEBSOCKETS=true bun run src/index.ts run \
  "Reply with only: received" \
  --pure --print-logs --log-level DEBUG --auto \
  --model openai/gpt-5.6-sol \
  -f /tmp/opencode-ws-{1..6}.png

On unpatched dev, this produces five ProviderResponseStreamError failures with code 1009: message too big before HTTP fallback succeeds. With this change, the same request immediately falls back to HTTP and returns the normal streamed response.

Testing

• bun test test/plugin/openai-ws.test.ts
• bun typecheck
• bunx prettier --check src/plugin/openai/ws.ts src/plugin/openai/ws-pool.ts test/plugin/openai-ws.test.ts