Fix excessive reads of /proc/mounts

[jenxie]

Short description

Optimizes read_mounts in Puppet::Util::SELinux by caching the parsed mount data using File.mtime('/proc/mounts'). This avoids reading the file thousands of times during a single catalog application.

Also includes a unit test to validate the caching behavior.

Fixes #619

Used Gemini to help debug the initial Ruby class variable state and structure the RSpec memoization test. All changes were manually verified and tested locally.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behavior changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[jenxie]

Created a couple of file resources with:

cat test_selinux.pp
Integer[1, 1000].each |$i| {
  file { "/tmp/test_file_${i}":
    ensure => present,
  }
}

On Rocky Linux 9.8 node with SELinux enabled temporarily applied the changes locally to /opt/puppetlabs/puppet/lib/ruby/vendor_ruby/puppet/util/selinux.rb , ran it with strace -e trace=open,openat,read -f puppet apply test_selinux.pp 2>&1 | grep "/proc/mounts" | wc -l

Before it resulted in 4211 and afterwards it results in 1.