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/vm.go

@@ -407,6 +407,46 @@ func (v *VM) BuildHeaders(name string, meta map[string]interface{}) (map[string]
 	return headers, nil
 }
 
+// TransformError executes the optional adapter.transform_error(status, body)
+// hook: per-source condensing of an upstream error response into a short
+// client-facing reason. ok=false means the adapter defines no hook (or it
+// failed) — the caller falls back to the generic Go-side condenser.
+func (v *VM) TransformError(name string, status int, body string) (reason string, ok bool, err error) {
+	p := v.pool(name)
+	if p == nil {
+		return "", false, fmt.Errorf("adapter %s not loaded", name)
+	}
+	w, err := p.acquire()
+	if err != nil {
+		return "", false, err
+	}
+	defer p.release(w)
+	L := w.L
+
+	L.SetTop(0)
+	defer L.SetTop(0)
+	L.GetGlobal(adapterGlobal)
+	if L.IsNil(-1) {
+		return "", false, nil
+	}
+	L.GetField(-1, "transform_error")
+	if !L.IsFunction(-1) {
+		return "", false, nil
+	}
+	L.SetTop(0)
+	L.GetGlobal(adapterGlobal)
+	L.GetField(-1, "transform_error")
+	L.PushInteger(int64(status))
+	L.PushString(body)
+	if callErr := L.Call(2, 1); callErr != nil {
+		return "", false, fmt.Errorf("transform_error: %w", callErr)
+	}
+	if L.Type(-1) != golua.LUA_TSTRING {
+		return "", false, nil
+	}
+	return L.ToString(-1), true, nil
+}
+
 func (v *VM) Endpoint(name string) string {
 	p := v.pool(name)
 	if p == nil {