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.

deploy/config.example.yaml

@@ -147,6 +147,7 @@ security:
       - "api.moonshot.cn"
       - "open.bigmodel.cn"
       - "api.minimaxi.com"
+      - "opencode.ai"
       - "generativelanguage.googleapis.com"
       - "cloudcode-pa.googleapis.com"
       - "*.openai.azure.com"