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/model/error_passthrough_rule.go

@@ -45,6 +45,7 @@ const (
 	PlatformZhipu       = domain.PlatformZhipu
 	PlatformDeepseek    = domain.PlatformDeepseek
 	PlatformMiniMax     = domain.PlatformMiniMax
+	PlatformOpenCodeGo  = domain.PlatformOpenCodeGo
 )
 
 // AllPlatforms 返回所有支持的平台列表
@@ -59,6 +60,7 @@ func AllPlatforms() []string {
 		PlatformZhipu,
 		PlatformDeepseek,
 		PlatformMiniMax,
+		PlatformOpenCodeGo,
 	}
 }