fix: cache plain arrays instead of Result objects to prevent deserialization errors

[pataar]

Summary

• Cache the result's plain array representation instead of the Result object itself, preventing __PHP_Incomplete_Class errors when Laravel deserializes cached entries
• Add regression test covering the exact scenario from the bug report

Closes #16

Test plan

• Existing tests pass (php vendor/bin/phpunit — 80 tests, 201 assertions)
• PHPStan passes at level max
• New test test_cache_returns_result_interface_not_incomplete_class verifies cached results deserialize correctly

[coveralls]

coverage: 100.0%. remained the same
when pulling 87155b5 on fix/cache-serialization-issue
into d9220f9 on main.