fix: prevent 500 errors from missing logger level check

[zaixiakongyiji]

Problem

Requests that reached normalize_gemini_request() failed with HTTP 500 because the custom Logger implementation did not provide the is_enabled_for() method used by the converter.

Fix

Add Logger.is_enabled_for() with the same level-threshold semantics as the existing logger, while preserving the debug log guard in normalize_gemini_request().

Validation

• python -m compileall -q log.py src tests/test_gemini_normalize.py
• Direct async smoke test for normalize_gemini_request() passed.
• Added a regression test covering normalization with the logger guard.

The full pytest suite could not be run in this environment because pytest is not installed in the available Python environments.