Fix DCO plugin trusted_apps not working without skip flags

[vigneshakaviki]

Summary

• Fix trusted_apps config having no effect when both SkipDCOCheckForMembers and SkipDCOCheckForCollaborators are false
• The filterTrustedUsers() call was gated behind config.SkipDCOCheckForMembers || config.SkipDCOCheckForCollaborators, but trusted_apps should work independently of those flags
• Add len(config.TrustedApps) > 0 to the condition so trusted apps (e.g. dependabot) are filtered even without the skip flags

Fixes #606

Root Cause

In handle() (pkg/plugins/dco/dco.go), filterTrustedUsers — which processes trusted_apps via trigger.TrustedUser() — was only invoked when skip flags were enabled:

// Before (broken):
if config.SkipDCOCheckForMembers || config.SkipDCOCheckForCollaborators {
    commitsMissingDCO, err = filterTrustedUsers(...)
}

// After (fixed):
if config.SkipDCOCheckForMembers || config.SkipDCOCheckForCollaborators || len(config.TrustedApps) > 0 {
    commitsMissingDCO, err = filterTrustedUsers(...)
}

Test plan

• Added test case: should skip dco check for trusted app even without skip flags — configures only TrustedApps: ["dependabot"] (no skip flags) and verifies a dependabot[bot] commit passes DCO
• All existing DCO tests pass (go test ./pkg/plugins/dco/...)

=== RUN   TestHandlePullRequest
--- PASS: TestHandlePullRequest (0.00s)
    --- PASS: .../should_skip_dco_check_for_trusted_app_even_without_skip_flags (0.00s)
    ... (17 other subtests all PASS)
PASS
ok  	sigs.k8s.io/prow/pkg/plugins/dco	0.019s

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• ✅ login: vigneshakaviki / name: vigneshakaviki (eabb696)

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Hi @vigneshakaviki. Thanks for your PR.

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[Prucek]

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Prucek, vigneshakaviki
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@vigneshakaviki: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-prow-integration	eabb696	link	true	/test pull-prow-integration

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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