test(inputs.tail): Add test coverage for plugin behavior

[skartikey]

Summary

Adds engine-level tests for the tail plugin that exercise the tailer behaviors the existing suite leaves uncovered: follow-append, rename/recreate and copytruncate rotation, delete-and-recreate, partial (unterminated) lines, symlinks, and offset resume across a restart. Each scenario runs under both the native watcher and polling (polling only on Windows) so coverage is independent of the watch implementation.

Two behaviors the current library does not satisfy on any platform are recorded as documented t.Skips rather than hidden, so the gaps stay visible:

• same-length copytruncate: the library flags a truncation only when the file size shrinks (prevSize > size in both watch/polling.go and watch/inotify.go), so a truncate-and-rewrite back to the same length is missed under both the native watcher and polling;
• mid-multiline resume: on stop the offset is taken from the last byte read rather than the last fully delivered entry, so resuming in the middle of a multi-line entry delivers it truncated.

A few other scenarios are skipped only on the specific watch-method/OS combinations the current library cannot handle (rename/recreate and delete/recreate under the native watcher, symlink target writes via the native watcher on macOS, and several cases under Windows polling); these still run on every other combination.

Intent is to serve as a fixed acceptance gate for the in-progress replacement of the unmaintained influxdata/tail library: the replacement should keep all scenarios green under both watch methods and ideally clear the two skipped behaviors.

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 #

[telegraf-tiger]

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. 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