server: Fixed speculative decoding stats to use #accepted \ #tested rather than #accepted \ #drafted

[jukofyork]

Fixes #14048

The existing printout of "draft acceptance rate" is misleading, as it is counting those tokens that were skipped due to the --draft-min setting:

                // keep track of total number of tokens generated in the draft
                slot.n_draft_total += draft.size();

                // ignore small drafts
                if (slot.params.speculative.n_min > (int) draft.size()) {
                    SLT_DBG(slot, "ignoring small draft: %d < %d\n", (int) draft.size(), slot.params.speculative.n_min);

                    continue;
                }

This PR just moves the count increment to after this test so that "draft acceptance rate" becomes #accepted \ #tested rather than #accepted \ #drafted.

As I said ion the other thread, we could add a separate #accepted \ #drafted stat, but this too would be misleading as there is code in speculative.cpp to reuse the drafted tokens...