kv: zero CommitTS when StartTS is auto-assigned in coordinators

[Copilot]

When StartTS == 0 and a coordinator auto-assigns it, any caller-provided CommitTS was still forwarded into dispatchTxn. Since the freshly-issued StartTS is generated from the HLC at dispatch time, a pre-supplied CommitTS can easily satisfy CommitTS <= StartTS, producing an invalid transaction or observing an arbitrary external timestamp into the clock.

Changes

• kv/coordinator.go: Zero reqs.CommitTS alongside reqs.StartTS assignment when StartTS == 0, so dispatchTxn generates both timestamps consistently.
• kv/sharded_coordinator.go: Same fix in the sharded path — clear CommitTS whenever the coordinator assigns StartTS.

This mirrors the existing redirect() logic, which already clears CommitTS when forwarding a txn without a StartTS:

// Before (both coordinators):
if reqs.IsTxn && reqs.StartTS == 0 {
    reqs.StartTS = c.nextStartTS()
    // CommitTS still forwarded — could be <= newly assigned StartTS
}

// After:
if reqs.IsTxn && reqs.StartTS == 0 {
    reqs.StartTS = c.nextStartTS()
    reqs.CommitTS = 0  // let dispatchTxn derive CommitTS from the real StartTS
}

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