feat(adapter): move per-source error condensing into transform_error hooks

Every upstream formats errors differently, which is adapter territory:
the protocol gains an optional transform_error(status, body) hook and all
built-in adapters implement their own envelope parsing (zen free-pool
labels, anthropic/gemini/ollama/mistral shapes, sensenova quota notes,
agentrouter WAF pages). The core keeps a single uniform fallback: when no
hook yields a reason clients get "api error <status>: unknown error" and
the raw body goes to server logs only.

internal/lua/adapters/sensenova.lua

@@ -105,4 +105,19 @@ function adapter.transform_stream_chunk(raw_chunk)
     return json.encode(unified)
 end
 
+-- 错误收敛：sensenova 为 OpenAI 风格 {error:{message,...}}；
+-- 配额类错误单独点出便于客户端识别重置周期。
+function adapter.transform_error(status, body)
+    local ok, resp = pcall(json.decode, body)
+    if not ok or type(resp) ~= "table" then return nil end
+    local e = resp.error
+    if type(e) ~= "table" then return nil end
+    if status == 429 and type(e.code) == "string"
+        and e.code == "insufficient_quota" then
+        return "workspace quota exhausted (resets periodically)"
+    end
+    if type(e.message) == "string" then return e.message end
+    return nil
+end
+
 return adapter