cli-plugins/hooks: fix max-message off-by-one

[fallintoplace]

Fixes #7005

What changed

This updates cli-plugins/hooks.ParseTemplate() to enforce the 10-message limit on the actual number of rendered messages instead of on the number of newline separators alone.

It also adds a regression test for the 11-line case.

Why it changed

The current implementation rejects only when strings.Count(out, "\n") > maxMessages and then returns strings.SplitN(out, "\n", maxMessages).

That makes the guard off by one: output with 10 newline characters contains 11 messages, but is still accepted. The extra rendered line is then folded into the last returned element.

Impact

Templates that render to more than 10 messages are now rejected consistently, including the 11-line / 10-newline case.

Valid templates keep the existing split behavior.

Root cause

The limit check counted separators instead of messages.

Validation

• GOPATH=$(mktemp -d) GO111MODULE=off go test github.com/docker/cli/cli-plugins/hooks via a temporary GOPATH layout rooted at this checkout

[thaJeztah]

Thank you for contributing! It looks like your commit message is missing a DCO sign-off,
causing the DCO check to fail.

We require all commit messages to have a Signed-off-by line with your name
and e-mail (see "Sign your work"
in the CONTRIBUTING.md in this repository), which looks something like:

Signed-off-by: YourFirstName YourLastName <yourname@example.org>

There is no need to open a new pull request, but to fix this (and make CI pass),
you need to amend the commit(s) in this pull request, and "force push" the amended
commit.

Unfortunately, it's not possible to do so through GitHub's web UI, so this needs
to be done through the git commandline.

You can find some instructions in the output of the DCO check (which can be found
in the "checks" tab on this pull request), as well as in the Moby contributing guide.

Steps to do so "roughly" come down to:

1. Set your name and e-mail in git's configuration:

   git config --global user.name "YourFirstName YourLastName"
   git config --global user.email "yourname@example.org"

   (Make sure to use your real name (not your GitHub username/handle) and e-mail)

2. Clone your fork locally

3. Check out the branch associated with this pull request

4. Sign-off and amend the existing commit(s)

   git commit --amend --no-edit --signoff

   If your pull request contains multiple commits, either squash the commits (if
   needed) or sign-off each individual commit.

5. Force push your branch to GitHub (using the --force or --force-with-lease flags) to update the pull request.

Let me know if you need help or more detailed instructions!

[fallintoplace]

@thaJeztah Thank you. I will check later today.