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.

tools/yaegi-check/main.go

@@ -50,14 +50,19 @@ const (
 )
 
 // excludedTopLevelDirs lists repo-root directories the GOPATH copy must
-// never include: build tooling, integration fixtures, planning docs, and
-// VCS metadata have nothing to do with the plugin package Yaegi imports.
+// never include: build tooling, integration fixtures, planning docs, VCS
+// metadata, and vendor (task-8's test-only testify dependency — _test.go
+// files are never interpreted by Yaegi, so vendor/ has nothing this check
+// needs, and copying its "go.yaml.in"-style nested module dirs into a
+// module-less GOPATH tree would otherwise confuse Yaegi's own import
+// resolution) have nothing to do with the plugin package Yaegi imports.
 var excludedTopLevelDirs = map[string]bool{
 	"tools":        true,
 	"integration":  true,
 	".superpowers": true,
 	"docs":         true,
 	".git":         true,
+	"vendor":       true,
 }
 
 func main() {