fix(secretstores.vault): Preserve sibling keys when setting a secret

[skartikey]

Summary

Setting a secret with telegraf secrets set wrote only the new key to the secret path, and Vault's Put replaces the whole secret at the path rather than merging, so every other key stored at that path was silently removed.

This reads the existing secrets at the path first and sets the new key on top of them before writing back. A not-found path is treated as empty so setting into a brand-new path still works, while any other read error aborts the write so a transient failure cannot overwrite existing secrets.

Checklist

• No AI generated code was used in this PR
• AI generated code used in this PR follows the InfluxData Policy on AI-Generated Code Contributions

Related issues

resolves #19287

[telegraf-tiger]

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☺️ 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