fix(tools): reject clusterSendMsgRT amount below two instead of printing NaN

[zjncs]

Motivation

mqadmin clusterSendMsgRT -a 1 cannot measure anything: the only send is the warm-up sample, elapsed stays 0, and the RT is computed as 0/0 = NaN. In tlog mode new BigDecimal(NaN) then throws NumberFormatException on the very first round, aborting the whole monitoring command.

Modifications

Reject an amount below two up front with a clear usage message, since at least two messages are required for a measured RT.

Verification

Fail-before (new test, run against the unpatched code):

ClusterSendMsgRTCommandTest#testAmountBelowTwoIsRejected
java.lang.AssertionError: ClusterSendMsgRTCommand command failed ; Infinite or NaN
(warm-up-only run computed RT = 0/0 = NaN and BigDecimal rejected it on the first round)

Pass-after:

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0 -- ClusterSendMsgRTCommandTest

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[RockteMQ-AI]

LGTM. Good defensive fix — rejecting amount < 2 up front prevents the NaN/NumberFormatException that would otherwise abort the monitoring command. Clear error message explains why.

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