[release-18.0] VtctldClient Reshard: add e2e tests to confirm CLI options and fix discovered issues. (#15353)

[rohit-nayak-ps]

Description

This is a backport of #15353

I had to redo this because #15363 refused to start CI workflows, apparently due to a Skip Label issue.

[vitess-bot]

Review Checklist

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General

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