backend/plugins: use structured logging in ListPlugins

[NAME-ASHWANIYADAV]

Fixes #5069

What this PR does

Refactors ListPlugins to replace raw fmt.Printf / fmt.Println calls with Headlamp's structured logger.Log, and removes the brittle os.Pipe stdout-capture technique from the test.

Changes

File	Change
backend/pkg/plugins/plugins.go	Replace 9 fmt.Printf/fmt.Println calls with logger.Log(logger.LevelInfo, ...) using structured key-value maps (plugin, type, directory)
backend/pkg/plugins/plugins_test.go	Delete captureOutput (os.Pipe hack), add t.Helper() to createPlugin, rewrite TestListPlugins to assert no-error directly

Why

• plugins.go was the only file in the entire backend still using fmt.Println for output - every other package uses the structured logger.
• The captureOutput test helper hijacked os.Stdout via os.Pipe, which is fragile and not goroutine-safe.
• Structured logs enable proper filtering and ingestion in Kubernetes pod log pipelines.

Testing

• go vet ./pkg/plugins/... - Clean
• go fmt ./pkg/plugins/... - No changes
• go test ./pkg/plugins/... -v - All tests pass

[NAME-ASHWANIYADAV]

/assign @sniok

[k8s-ci-robot]

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