Infinite recursion on missing implementation

@mhoemmen @crtrott @fnrizzi

If an algorithm is called with execution policy that has identical mapping (i.e. execpolicy_mapper returns same type as it gets) - like kokkos_exec<ExecSpace> - but implementation (or that particular overload) is missing, the dispatcher enters infinite recursion. It results in SEGFAULT crash in Debug builds or hangs in Release builds (e.g. see the hanging tests cancelled after 25m).

This happens because is_custom_<ALG>_avail checks mistake the dispatcher routine for requested custom implementation (as - with id policy mapping - their function signatures are identical).

Start by tracing the dispatcher and is_custom__avail described in the issue, using the linked hanging test run as the failure reference. Confirm how identical execution-policy mapping selects the dispatcher again, then add coverage showing that a missing implementation terminates cleanly instead of recursing or hanging.