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_test.go

@@ -17,5 +17,6 @@ func TestAllPlatformsIncludesEveryConcretePlatform(t *testing.T) {
 		"zhipu",
 		"deepseek",
 		"minimax",
+		"opencode_go",
 	}, AllPlatforms())
 }