Add dependency injection coding standards

[cameron1729]

Summary

This PR adds guidance for using Moodle's dependency injection container without introducing service location patterns.

It introduces two checks:

• A coding standard error when application code receives the DI container as a dependency (this is explicitly prohibited in developer documentation)
• An opt-in warning in moodle-extra, for when a class performs direct \core\di lookups

Rationale

Dependency injection means that classes declare their immediate collaborators. Injecting the container instead gives a class access to arbitrary services, hides its real dependencies, and effectively turns the container into a service locator.

Direct container lookups inside classes are also usually service location. However, container access is legitimate at composition roots and other application boundaries, so this is provided as a warning through Moodle Extra rather than an unconditional error.

Related work

This change was prompted by the propsed Calendar API migration in MDL-89216, but the coding standard rules added here apply generally to the existing DI container and can be merged independently.

Supporting improvements to the developer documentation are also in progress here: moodle/devdocs#1640

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.34%. Comparing base (b60df78) to head (a9d9ef5).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #228      +/-   ##
============================================
+ Coverage     98.27%   98.34%   +0.06%     
- Complexity      994     1050      +56     
============================================
  Files            42       44       +2     
  Lines          2961     3082     +121     
============================================
+ Hits           2910     3031     +121     
  Misses           51       51              

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