Unify fallback and unimplemented API error handling

- Add a shared common error response schema and unknown error code for business-level fallbacks
- Return business error payloads for router NoRoute and NoMethod fallbacks
- Align the existing event list fallback response with the shared error structure
- Introduce an explicit unimplemented API error type instead of relying on formatted strings
- Make /api batch dispatch return per-item status 600 error entries for unimplemented modules and actions without failing the whole batch

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/utils/error.go

@@ -0,0 +1,37 @@
+package honokautils
+
+import "errors"
+
+type UnimplementedError struct {
+	Kind   string
+	Module string
+	Name   string
+}
+
+func (e *UnimplementedError) Error() string {
+	if e == nil {
+		return "unimplemented"
+	}
+	return "unimplemented " + e.Kind + ": " + e.Module + ": " + e.Name
+}
+
+func NewUnimplementedModuleError(module string) error {
+	return &UnimplementedError{
+		Kind:   "api module",
+		Module: module,
+		Name:   module,
+	}
+}
+
+func NewUnimplementedActionError(module string, action string) error {
+	return &UnimplementedError{
+		Kind:   "action",
+		Module: module,
+		Name:   action,
+	}
+}
+
+func IsUnimplementedError(err error) bool {
+	var target *UnimplementedError
+	return err != nil && errors.As(err, &target)
+}

internal/utils/request.go

@@ -1,4 +1,4 @@
-package utils
+package honokautils
 
 import (
 	"encoding/json"

internal/utils/serverdata.go

@@ -1,4 +1,4 @@
-package utils
+package honokautils
 
 import (
 	"encoding/json"