Implement teardown contract for ESPFetch

[zekageri]

Summary

• replace the public runtime-state accessor with isInitialized() const (no compatibility alias)
• harden teardown lifecycle by making deinit() idempotent/pre-init-safe and signaling in-flight workers to abort during teardown
• keep destructor teardown delegated through deinit() and gate request enqueue on initialization state
• expand lifecycle tests for pre-init deinit, repeated deinit, and init -> deinit -> init
• document explicit teardown usage in README and example sketch

Testing

• could not run local build/test tools in this environment (cmake and platformio are not installed)