feat(opencode): add OpenCode platform with Zen and GO account types

Keep the internal platform ID opencode_go. Product surface is OpenCode
with account_mode zen|go, model-aware routing across Chat Completions,
Responses, and Anthropic Messages, and X-OpenCode-Session derived from
prompt_cache_key / metadata.user_id so prompt cache can hit.

Unmatched models fall back to /v1/chat/completions. Related to #6742.

backend/internal/handler/admin/channel_handler.go

@@ -644,6 +644,7 @@ var platformToLiteLLMProvider = map[string]string{
 	service.PlatformZhipu:       "zhipu",
 	service.PlatformDeepseek:    "deepseek",
 	service.PlatformMiniMax:     "minimax",
+	service.PlatformOpenCodeGo:  "opencode-go",
 }
 
 // SyncPricingModels 返回 LiteLLM 定价目录中指定平台的最新模型列表