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