fix(opencode): retry empty unknown responses

[opencode-agent]

Summary

• detect provider attempts that finish with an unknown reason without producing text, reasoning, or tool calls
• route those attempts through the existing session retry policy and status updates
• retry the same request instead of silently completing an empty assistant turn
• preserve prior step output while recording the empty attempt and recovered response

Testing

• bun test test/session/processor-effect.test.ts (16 passed)
• bun test test/session/retry.test.ts (33 passed)
• targeted test/cli/run/run-process.test.ts empty-response retry cases (2 passed)
• bun x prettier --check packages/opencode/src/session/processor.ts packages/opencode/src/session/retry.ts packages/opencode/test/session/processor-effect.test.ts packages/opencode/test/cli/run/run-process.test.ts
• CI typecheck passed

Requested by: @rekram1-node (Aiden Cline via Slack)