sampler : remove "full-context windows" from history-based samplers (#26524)

* Resolve -1 to 1024 instead of ctx-len for samplers

Because of backend-sampling we initialize samplers before the complete
llama_context is there. Therefore, we cannot infer the resolved context
length yet at the time we construct the samplers.

* Shared default of 64 for history-based samplers, remove context_size

common/sampling.h

@@ -39,8 +39,7 @@ struct common_sampler;
 // note: can mutate params in some cases
 struct common_sampler * common_sampler_init(
         const struct llama_model * model,
-        struct common_params_sampling & params,
-        int32_t n_ctx = 0);
+        struct common_params_sampling & params);
 
 void common_sampler_free(struct common_sampler * gsmpl);