ci: enforce patch coverage of changed lines

[TheWitness]

What

Brings this plugin's CI in line with plugin_thold by enforcing patch
coverage — coverage of the production PHP lines a pull request actually
changes — and cleans up an inert env var.

Changes:

• Add tests/bin/patch-coverage.php (mirrors plugin_thold).
• Add an Enforce coverage of changed lines CI step, gated to
  pull_request events, that fails when production lines a PR adds/modifies
  are left uncovered by the Pest suite.
• Set fetch-depth: 0 on the plugin checkout so the gate can diff against
  the PR base commit.
• Remove COMPOSER_ROOT_VERSION: 1.3.0-dev from the Pest step. It only
  influences Composer root-version detection, which does not run in that
  step, so it was inert.

Why

Whole-file coverage is not meaningful for a Cacti plugin: most code only
runs inside a live Cacti, so the repository figure sits near zero regardless
of test quality. What is enforceable — and what a reviewer cares about — is
that the lines a change introduces are exercised. This is the same gate
plugin_thold already uses; this PR is part of harmonizing it across the
plugin fleet.

Notes

• The gate only runs on pull requests. On this PR it has no production PHP
  lines to measure (only workflow YAML and the tests/ helper changed), so
  it reports "no measured lines changed" and passes.
• patch-coverage.php's $unmeasured_allowlist starts empty; entry points
  that cannot be loaded into the isolated unit process can be added there
  per repo, each with a one-line justification, as the need arises.

[Copilot]

Copilot review overview

🔵 Needs a closer look

Renamed PHP files can bypass the gate, while edits to production files excluded from Clover can fail it regardless of their tests.

Review effort: Balanced
Findings: None

What changed in this PR

This PR adds a changed-line coverage gate to the Syslog plugin’s pull-request CI, following the approach used by plugin_thold.

Changes:

• Add a helper that compares changed PHP lines with Clover coverage.
• Run the gate on pull requests, fetch the base history, and remove an inert Composer environment variable.

File	Description
tests/​bin/​patch-coverage.php	Calculates coverage for changed production PHP lines.
.github/​workflows/​plugin-ci-workflow.yml	Adds the pull-request gate and fetches the history it needs.

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