feat: add MCP enums, ToolError, and Caller

client/caller.go

@@ -0,0 +1,18 @@
+package client
+
+import (
+	"context"
+	"encoding/json"
+)
+
+// Caller invokes an MCP tool by name and returns its JSON result.
+type Caller interface {
+	Call(ctx context.Context, name string, args map[string]any) (json.RawMessage, error)
+}
+
+// Func adapts a function to Caller so tests can inject a stub.
+type Func func(ctx context.Context, name string, args map[string]any) (json.RawMessage, error)
+
+func (f Func) Call(ctx context.Context, name string, args map[string]any) (json.RawMessage, error) {
+	return f(ctx, name, args)
+}