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/forward_assertions.go

@@ -0,0 +1,16 @@
+package assert
+
+// Assertions provides assertion methods around the
+// TestingT interface.
+type Assertions struct {
+	t TestingT
+}
+
+// New makes a new Assertions object for the specified TestingT.
+func New(t TestingT) *Assertions {
+	return &Assertions{
+		t: t,
+	}
+}
+
+//go:generate sh -c "cd ../_codegen && go build && cd - && ../_codegen/_codegen -output-package=assert -template=assertion_forward.go.tmpl -include-format-funcs"