chore(deps): vendor testify

Add github.com/stretchr/testify (test-only) as a direct dependency and
vendor it, matching the traefikoidc precedent: _test.go files are never
Yaegi-interpreted, so vendoring a test-only dependency does not affect the
plugin's runtime under Traefik or the catalog analyzer.

tools/yaegi-check/main.go: add vendor to the harness's repo-copy exclude
list, since vendor/'s nested module layout (go.yaml.in/yaml/v3 alongside
testify) has nothing to do with the plugin package Yaegi imports.

Verified: make yaegi-check stays green; go build/vet/test unaffected.

vendor/github.com/stretchr/testify/assert/yaml/yaml_custom.go

@@ -0,0 +1,24 @@
+//go:build testify_yaml_custom && !testify_yaml_fail && !testify_yaml_default
+
+// Package yaml is an implementation of YAML functions that calls a pluggable implementation.
+//
+// This implementation is selected with the testify_yaml_custom build tag.
+//
+//	go test -tags testify_yaml_custom
+//
+// This implementation can be used at build time to replace the default implementation
+// to avoid linking with [go.yaml.in/yaml/v3].
+//
+// In your test package:
+//
+//		import assertYaml "github.com/stretchr/testify/assert/yaml"
+//
+//		func init() {
+//			assertYaml.Unmarshal = func (in []byte, out interface{}) error {
+//				// ...
+//	     			return nil
+//			}
+//		}
+package yaml
+
+var Unmarshal func(in []byte, out interface{}) error

vendor/github.com/stretchr/testify/assert/yaml/yaml_default.go

@@ -0,0 +1,36 @@
+//go:build !testify_yaml_fail && !testify_yaml_custom
+
+// Package yaml is just an indirection to handle YAML deserialization.
+//
+// This package is just an indirection that allows the builder to override the
+// indirection with an alternative implementation of this package that uses
+// another implementation of YAML deserialization. This allows to not either not
+// use YAML deserialization at all, or to use another implementation than
+// [go.yaml.in/yaml/v3] (for example for license compatibility reasons, see [PR #1120]).
+//
+// Alternative implementations are selected using build tags:
+//
+//   - testify_yaml_fail: [Unmarshal] always fails with an error
+//   - testify_yaml_custom: [Unmarshal] is a variable. Caller must initialize it
+//     before calling any of [github.com/stretchr/testify/assert.YAMLEq] or
+//     [github.com/stretchr/testify/assert.YAMLEqf].
+//
+// Usage:
+//
+//	go test -tags testify_yaml_fail
+//
+// You can check with "go list" which implementation is linked:
+//
+//	go list -f '{{.Imports}}' github.com/stretchr/testify/assert/yaml
+//	go list -tags testify_yaml_fail -f '{{.Imports}}' github.com/stretchr/testify/assert/yaml
+//	go list -tags testify_yaml_custom -f '{{.Imports}}' github.com/stretchr/testify/assert/yaml
+//
+// [PR #1120]: https://github.com/stretchr/testify/pull/1120
+package yaml
+
+import goyaml "go.yaml.in/yaml/v3"
+
+// Unmarshal is just a wrapper of [go.yaml.in/yaml/v3.Unmarshal].
+func Unmarshal(in []byte, out interface{}) error {
+	return goyaml.Unmarshal(in, out)
+}

vendor/github.com/stretchr/testify/assert/yaml/yaml_fail.go

@@ -0,0 +1,17 @@
+//go:build testify_yaml_fail && !testify_yaml_custom && !testify_yaml_default
+
+// Package yaml is an implementation of YAML functions that always fail.
+//
+// This implementation can be used at build time to replace the default implementation
+// to avoid linking with [go.yaml.in/yaml/v3]:
+//
+//	go test -tags testify_yaml_fail
+package yaml
+
+import "errors"
+
+var errNotImplemented = errors.New("YAML functions are not available (see https://pkg.go.dev/github.com/stretchr/testify/assert/yaml)")
+
+func Unmarshal([]byte, interface{}) error {
+	return errNotImplemented
+}