Implement teardown contract for ESPWebPush

[zekageri]

Summary

• replace initialized() with isInitialized() and remove compatibility aliasing
• harden deinit() to fully converge teardown (worker task, queue drain/delete, crypto state reset, runtime key/config release) even for partial states
• make worker task handle synchronization explicit with atomics to avoid teardown races
• add lifecycle tests for pre-init deinit, idempotent deinit, re-init, and destructor teardown
• update README/example/changelog to document explicit teardown usage and new API naming

Validation

• cmake -S . -B build (fails in this environment: cmake not installed)
• pio --version (fails in this environment: PlatformIO not installed)