fix: stop the requested backend child

[ruslandoga]

Summary

• Stop only the backend child whose ID was requested.
• Cover stopping a non-first backend child and handling a missing backend ID without disturbing existing children.

Why

The piped Enum.find_value/3 call passed the apparent predicate in the default-value position and &elem(&1, 0) as the mapper. The mapper returned the first supervisor child ID, which was truthy, so enumeration stopped before comparing the requested backend ID.

Test-first result

Before changing production code:

• mix test test/logflare/backends_test.exs:827 test/logflare/backends_test.exs:887
• Result: 2 tests, 2 failures (99 excluded). The requested non-first backend child remained alive, and the missing-ID call returned :ok.

Verification

• mix test test/logflare/backends_test.exs:827: 1 test, 0 failures (99 excluded).
• mix test test/logflare/backends_test.exs: 97 tests, 0 failures (3 excluded).

[ruslandoga]

Converting to draft until I simplify the test...