[BUG]: Type validation failed for billing.summary SSE event from SiliconFlow-compatible providers

Description

When using a provider that injects billing metadata into the SSE stream (e.g., SiliconFlow-compatible APIs), opencode crashes with a type validation error because the response schema only accepts choices array or error object, but billing summary events have neither.

Error

Type validation failed: Value: {"billing":{"source":"request","occurred_month":"2026-07","request":{"success":true,"stream":true,"tokens":{"input_tokens":29816,"cache_creation_input_tokens":0,"cache_read_input_tokens":0,"output_tokens":873,"reasoning_tokens":516,"total_tokens":30689},"cost_cny":{"input":"0.2087","cache_creation":"0.0000","cache_read":"0.0000","output":"0.0367","reasoning":"0.0000","request":"0.0000","duration":"0.0000","quantity":"0.0000","parameter":"0.0000","total":"0.2454"}},"api_key_period":{"period":{"usage_type":"","reset_cycle":"","period_key":""},"stats":{"request_count":0,"success_count":0,"error_count":0,"stream_request_count":0},"tokens":{"input_tokens":0,"cache_creation_input_tokens":0,"cache_read_input_tokens":0,"output_tokens":0,"reasoning_tokens":0,"total_tokens":0},"cost_cny":{"total":""}}},"object":"billing.summary"}.
Error message: [
  {
    "code": "invalid_union",
    "errors": [
      [
        {
          "expected": "array",
          "code": "invalid_type",
          "path": ["choices"],
          "message": "Invalid input: expected array, received undefined"
        }
      ],
      [
        {
          "expected": "object",
          "code": "invalid_type",
          "path": ["error"],
          "message": "Invalid input: expected object, received undefined"
        }
      ]
    ],
    "path": [],
    "message": "Invalid input"
  }
]

Root Cause

The SSE stream response validation uses a union type expecting either:

1. A successful response with a choices array, OR
2. An error response with an error object

Providers like SiliconFlow inject extra SSE events with "object": "billing.summary" at the end of the stream. These are not part of the OpenAI spec and cause a hard validation failure instead of being silently ignored.

Expected Behavior

Unknown SSE event object types (e.g., billing.summary) should be silently ignored rather than throwing a validation error.

Steps to Reproduce

1. Configure opencode with a SiliconFlow-compatible provider
2. Send any chat request
3. Observe the Type validation failed error in the output

Environment

• Provider: SiliconFlow-compatible API (injects billing.summary into SSE stream)