Implement teardown contract for ESPLogger

[zekageri]

Summary

• harden ESPLogger::deinit() so teardown is safe and idempotent, including callback and buffer reset plus teardown-time flush
• fix init failure cleanup when sync task creation fails to avoid mutex lifetime misuse
• skip sync staging work when no sync callback is registered
• add lifecycle tests for pre-init deinit, double deinit, re-init, callback teardown behavior, and destructor-driven teardown
• document explicit deinit usage in README and examples

Validation

• g++ -std=c++17 -I./src -I./test -I./test/stubs ./src/esp_logger/logger.cpp ./test/logger_test_stubs.cpp ./test/logger_tests.cpp -o /tmp/logger_tests && /tmp/logger_tests
• cmake -S . -B build && cmake --build build && ctest --test-dir build --output-on-failure (fails in this environment: cmake not installed)