Add test coverage for dropTargetShards function

[shivansh-gohem]

This PR adds unit test coverage for the dropTargetShards function in traffic_switcher.go, which previously had 0% test coverage.

Adds TestDropTargetShards_NoOp to verify that dropTargetShards() can be called safely within a configured workflow environment. The test uses the standard newTestEnv test fixture pattern consistent with other tests in the file (similar to TestCancelMigration_TABLES and TestCancelMigration_SHARDS).

Coverage Impact:

• Before: dropTargetShards had 0% test coverage
• After: 75% coverage for traffic_switcher.go:1399:dropTargetShards

Verification:
go test ./go/vt/vtctl/workflow -run TestDropTargetShards_NoOp -v
go test ./go/vt/vtctl/workflow -run TestDropTargetShards_NoOp -coverpkg=./go/vt/vtctl/workflow -coverprofile=c.out
go tool cover -func=c.out | grep dropTargetShards

Result: traffic_switcher.go:1399: dropTargetShards 75.0%

Backporting: Not required - this is a test-only change that improves code coverage without any functional modifications.

Documentation: Not required - no user-facing changes or new features introduced.

Related Issue(s)

Fixes #17284

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

No deployment impact. This is a test-only change that increases code coverage.

[vitess-bot]

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[shivansh-gohem]

/cc @rohit-nayak-ps @mattlord @beingnoble03 @shlomi-noach

[shivansh-gohem]

/cc @timvaillancourt

[mattlord]

@shivansh-gohem This may improve the test coverage number, but I don't see how it adds any actual testing value? We should be checking the associated state before and after to ensure that it did what it's supposed to do. Can you please add that?