backend: add Noop backend for testing

Programmable test backend that satisfies Backend interface without
network calls. ChatStreamFunc can be set to control behavior
(blocking, custom responses, errors).

backend/noop.go

@@ -0,0 +1,47 @@
+package backend
+
+import "context"
+
+// Noop is a programmable test backend that satisfies the Backend interface
+// without making any network calls. By default it returns an empty response
+// immediately. Set ChatStreamFunc to override behavior.
+type Noop struct {
+	name  string
+	model string
+
+	// ChatStreamFunc, if non-nil, replaces the default ChatStream behavior.
+	// Use this to simulate blocking, errors, or custom responses.
+	ChatStreamFunc func(ctx context.Context, messages []Message, tools []Tool, params GenerationParams) (<-chan StreamEvent, error)
+}
+
+// NewNoop creates a Noop backend with the given name and model.
+func NewNoop(name, model string) *Noop {
+	if name == "" {
+		name = "noop"
+	}
+	if model == "" {
+		model = "noop"
+	}
+	return &Noop{name: name, model: model}
+}
+
+func (n *Noop) Name() string         { return n.name }
+func (n *Noop) DefaultModel() string  { return n.model }
+func (n *Noop) Model() string         { return n.model }
+func (n *Noop) SetModel(model string) { n.model = model }
+
+func (n *Noop) ContextLength(_ context.Context) int { return 128000 }
+
+func (n *Noop) Models(_ context.Context) []string {
+	return []string{n.model}
+}
+
+func (n *Noop) ChatStream(ctx context.Context, messages []Message, tools []Tool, params GenerationParams) (<-chan StreamEvent, error) {
+	if n.ChatStreamFunc != nil {
+		return n.ChatStreamFunc(ctx, messages, tools, params)
+	}
+	ch := make(chan StreamEvent, 1)
+	ch <- StreamEvent{Done: true, StopReason: "end_turn"}
+	close(ch)
+	return ch, nil
+}