Add tests for webhook sender and stageLogPersister

[AshishSingh1503]

What this PR does:
Adds missing unit tests for two previously untested files in pkg/:

• pkg/app/piped/notifier/webhook_test.go — tests for newWebhookSender, sendEvent (using httptest.Server), Notify buffering, Run loop cancellation, and Close draining remaining events.
• pkg/app/piped/logpersister/stagelogpersister_test.go — tests for all log severity methods (Info, Infof, Success, Successf, Error, Errorf, Write), index increment ordering, all branches of isStale, and Complete timeout/success paths.

Why we need it:
These files had zero test coverage. Adding tests improves confidence when refactoring and helps catch regressions.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change: No user-facing change. Test-only addition.
• Is this breaking change: No
• How to migrate (if breaking change): N/A