fix(config): preserve unresolved env vars during mixed substitution

[WyRainBow]

Fix mixed substitution so unresolved ${VAR} is preserved when defaults appear earlier.

[telegraf-tiger]

Thanks so much for the pull request!
🤝 ✒️ Just a reminder that the CLA has not yet been signed, and we'll need it before merging. Please sign the CLA when you get a chance, then post a comment here saying !signed-cla

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_arm64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

👍 This pull request doesn't change the Telegraf binary size

📦 Click here to get additional PR build artifacts

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

[srebhan]

@WyRainBow please restore the PR description template, especially the AI section and the structure! This is required so we can review this PR. Furthermore, it allows us automatically react on things, e.g. close related issues automatically with merging the PR...

Besides of the above, I believe this issue should be fixed upstream (see compose-spec/compose-go#856), shouldn't it?

[WyRainBow]

@srebhan Sorry, I missed the upstream issue.

I’ll update this PR to restore the full description template (including the AI section and structure) as soon as possible.

I’m also investigating whether master already supports this behavior; if this is a library gap in compose-go, I’ll follow up upstream and keep this PR aligned.

Thanks a lot for the guidance.

[WyRainBow]

Thanks for the review and guidance.

I traced this further and confirmed the root cause is in compose-go option propagation during nested substitution.

I opened an upstream fix PR here: compose-spec/compose-go#861

Given that, I’m closing this Telegraf workaround PR in favor of the upstream fix path. Once it is merged and released, I can follow up with a dependency bump in Telegraf if needed.