fix: register theming context processor on CONTEXT_PROCESSORS for Verawood

[jignaciopm]

Description

From Verawood onwards, edx-platform builds its settings with the new Derived settings framework (openedx/envs/common.py). As part of that change, each template engine's context_processors is no longer a list but a Derived value:

TEMPLATES = [
    { 'NAME': 'django', ... 'context_processors': Derived(lambda settings: settings.CONTEXT_PROCESSORS) },
    { 'NAME': 'mako',   ... 'context_processors': Derived(lambda settings: settings.CONTEXT_PROCESSORS) },
]

plugin_settings() registered the theming context processor by mutating those entries directly:

settings.TEMPLATES[1]['OPTIONS']['context_processors'].append(eox_configuration_path)

On Verawood .append() / in run against a Derived object (which is not a list and not iterable), raising AttributeError/TypeError that was caught and only logged. The context processor was therefore never registered, leaving the theming template variable undefined and making every legacy Mako/Django page fail with:

AttributeError: 'Undefined' object has no attribute 'options'

Fix

Register the context processor on settings.CONTEXT_PROCESSORS instead. Both the Django and Mako engines derive their context_processors from this setting, so the change works across releases:

• Earlier releases point each engine's context_processors at the same CONTEXT_PROCESSORS list, so appending to it reaches both engines.
• Verawood and later resolve their Derived values to settings.CONTEXT_PROCESSORS when derive_settings() runs, picking up the appended processor.

A fallback is kept for releases that don't expose a CONTEXT_PROCESSORS setting (registers on each engine whose context_processors is still a plain list).

No support for earlier releases is dropped.

• User roles impacted: Operator (theming on Verawood deployments).
• Config changes: none required by operators; registration is automatic.

Testing instructions

1. On a Verawood environment with eox-theming installed and a comprehensive theme enabled (e.g. bragi/css-runtime), load any legacy Mako page (for example /404 or the login page).

   • Before: HTTP 500, AttributeError: 'Undefined' object has no attribute 'options'.
   • After: the page renders and theming.options(...) resolves.

2. Run the unit tests:

   make test-python
   # or, focused:
   DJANGO_SETTINGS_MODULE=eox_theming.settings.test pytest eox_theming/tests/test_settings.py

   New tests in eox_theming/tests/test_settings.py cover: registration via CONTEXT_PROCESSORS (Verawood/Derived), idempotency, the legacy shared-list path, and the no-CONTEXT_PROCESSORS fallback.

Additional information

• Bumps version 10.1.0 → 10.1.1, updates CHANGELOG.md (Fixed) and the README compatibility notes (Verawood >= 10.1.1).
• Adds a README note documenting the Verawood rename of _make_mako_template_dirs → make_mako_template_dirs (now in openedx.envs.common).
• v10.1.0 declared Verawood support but only updated dependencies/CI; this PR fixes the actual runtime regression.

Tested in a remote environment

Checklist for Merge

• Tested in a remote environment
• Updated documentation
• Rebased master/main
• Squashed commits

[MaferMazu]

This looks good to me. Thanks ✨