Gate SRU with clear warning in the description (Infra)

[Hook25]

Description

In order to avoid changing the sru testplan unintentionally, this PR introduces a validation mechanism that requires that if the sru testplan is touched by a pr, then the description must contain the following text:

WARNING: This modifies com.canonical.certification::sru-server

or

WARNING: This modifies com.canonical.certification::sru

The action lists the differences that the PR introduces for each sru testplan in the log, and fails if the text is not found

Resolved issues

Fixes: CHECKBOX-1512

Documentation

The action describes what to do when it fails, requiring the user to update the description due to the change to sru.

Tests

No change to sru, no error (note the two no diff): https://github.com/canonical/checkbox/actions/runs/10165387577/job/28113046601
Change to sru but no text in the description (note, the diff is the inclusion in the sru-server testplan of the ubuntu-desktop-recommands job): https://github.com/canonical/checkbox/actions/runs/10165493766/job/28113405735
Change to sru and text in the description (note how the diff is still shown, so we can audit it, but the workflow doesn't fail as the description was changed at that time to include the warning): https://github.com/canonical/checkbox/actions/runs/10165700224/job/28114127742

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.12%. Comparing base (c09a4ce) to head (a329c58).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1380   +/-   ##
=======================================
  Coverage   45.12%   45.12%           
=======================================
  Files         366      366           
  Lines       39058    39058           
  Branches     6607     6607           
=======================================
  Hits        17626    17626           
  Misses      20758    20758           
  Partials      674      674           

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[pieqq]

Just one request, but otherwise it looks good to me!

[pieqq]

Thanks!