fix(agent): Error out if configuration directory is not readable

[srebhan]

Summary

Currently Telegraf will silently ignore any configuration directory specified via command-line if this directory cannot be read by the Telegraf user. This is bad because users will see missing plugins but get no indication whatsoever on what is going on.

This PR will error out if the directory cannot be read.

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 #16600

[skartikey]

@srebhan Thanks for the fix! The change to error out on unreadable config directories is solid.

The test failures on Linux and Windows CI are likely because TestLoadConfigurationsPermissions relies on Unix file permission enforcement, which doesn't hold when running as root (common in Docker-based CI) or on Windows. Adding skip guards should fix it:

  if runtime.GOOS == "windows" {                                                                                                                                                                                   
      t.Skip("Unix file permissions not supported on Windows")
  }
  if os.Getuid() == 0 {
      t.Skip("Cannot test permission restrictions as root")                                                                                                                                                        
  }   

[skartikey]

@srebhan, some spelling typos.

[skartikey]

@srebhan Thanks!

[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

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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