fix(inputs.modbus): Split large request correctly at field borders

[srebhan]

Summary

The modbus plugin splits large requests to adhere to the maximum request size defined by the Modbus spec. Currently, the plugin does so by splitting at the maxBatchSize limit which leads to issues if this limit does not align with a field border.

This PR fixes the code to split the request at a field border, avoiding device addressing errors and invalid field offsets. The PR additionally plays safe when it comes to holding and input offset computation by casting the computed offset to the next higher datatype before multiplication to avoid overflows.

Checklist

• No AI generated code was used in this PR
• Unit-test for issue was added

Related issues

resolves #14387

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

☺️ This pull request doesn't significantly change the Telegraf binary size (less than 1%)

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Artifact URLs

DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_arm64.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz	windows_i386.zip
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz

[h0d3nt3uf3l]

Test the fix was successful. Problem solved for me.