[log] Add ESPHOME_DEBUG_ASSERT for log function invariants

Add ESPHOME_DEBUG_ASSERT macro that only fires when ESPHOME_DEBUG is
defined. Use it to assert global_logger is not null in log functions.
Enable ESPHOME_DEBUG in C++ unit test builds so these assertions are
active during testing but have zero cost in production firmware.

script/cpp_unit_test.py

@@ -100,6 +100,7 @@ def create_test_config(config_name: str, includes: list[str]) -> dict:
                 "build_flags": [
                     "-Og",  # optimize for debug
                     "-DUSE_TIME_TIMEZONE",  # enable timezone code paths for testing
+                    "-DESPHOME_DEBUG",  # enable debug assertions
                 ],
                 "debug_build_flags": [  # only for debug builds
                     "-g3",  # max debug info