common : add common_speculative_is_compat() (#19270)
* llama : add llama_memory_can_rm_suffix()
* Revert "llama : add llama_memory_can_rm_suffix()"
This reverts commit d30e59b62a15ef4266a6503e3f4eba770aec001b.
* spec : check if the target context is compatible for spec decoding
