Do not configure application root logging on dm_control import

[sylvesterkaczmarek]

Fixes #314.

Summary

• Avoid Abseil's implicit logging.basicConfig call during import-time OpenGL backend selection and MuJoCo bindings initialization.
• Use named standard-library loggers for those informational status messages, without installing a root logger handler.
• Preserve the caller's ability to configure logging.basicConfig after importing dm_control.mjcf, and avoid changing existing configured handlers.
• Add fresh-interpreter subprocess regression tests for both scenarios.

Verification

• Reproduced the problem on unchanged upstream main: importing dm_control.mjcf installs a root StreamHandler and prevents a later logging.basicConfig from applying the application's desired format.
• With this change the import leaves the root logger without handlers, and the user's format is applied correctly.
• 55 tests and 2 subtests passed across the new import logging tests, renderer base tests and PyMJCF element tests.
• Ruff selected checks (E4,E7,E9,F except existing F401 reexports), Python compilation and git diff --check passed.
• Imported against the local MuJoCo 3.15 Python runtime; no live GPU rendering was tested.

This change is limited to logging during module initialization and does not change runtime MuJoCo rendering behavior.