Move custom frontend and load all for all jobs (bugfix)

[Hook25]

Description

Previous implementation only loaded envvars per provider, effectively binding them to the providers they target. This was a mistake, as that means that providers can't rely on each other and resources like executables are basically useless with this new architecture.

This adresses this flaw by making all custom frontend variables apply to all jobs, as they should.

Resolved issues

N/A

Documentation

N/A

Tests

Added/moved tests to the new module.

Tested this locally by building checkbox24 and a custom-frontend provider. Snap trying both. Creating a new executable (touch a; chmod +x a) in the custom frontend /usr/bin, adding the following job to Checkbox to a random pxu and triggering it, it works.

id: test_path
flags: simple
requires: executable.name == 'a'
command:
  echo "it works :)"
_summary: Test that path includes all custom frontends

Command:

checkbox24.checkbox run test_path

[codecov]

Codecov Report

❌ Patch coverage is 96.77419% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.90%. Comparing base (851f35a) to head (10d4be4).
⚠️ Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
.../plainbox/impl/secure/providers/custom_frontend.py	94.52%	3 Missing and 1 partial ⚠️
checkbox-ng/plainbox/impl/execution.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2457      +/-   ##
==========================================
+ Coverage   58.87%   58.90%   +0.03%     
==========================================
  Files         476      478       +2     
  Lines       48004    48042      +38     
  Branches     8568     8580      +12     
==========================================
+ Hits        28260    28298      +38     
  Misses      18853    18853              
  Partials      891      891              

Flag	Coverage Δ
checkbox-ng	76.09% <96.77%> (+0.04%)	⬆️

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[p-gentili]

Was looking at the documentation, https://github.com/canonical/checkbox/blob/main/docs/how-to/custom-frontend-envvars.rst. You might want to rephrase "accessed globally by all tests in a frontend" with "accessed globally by all tests in a frontend regardless of which provider the test comes from". Or simple "accessed globally by all tests."

Testing now...