Coward Micro AI: add optional attack_if_trapped key

If set, the unit attacks the weakest enemy next to which it ends up at
the end of its move.

data/ai/micro_ais/cas/ca_coward.lua

@@ -83,6 +83,26 @@ function ca_coward:execution(ai, cfg)
     AH.movefull_stopunit(ai, coward, best_hex[1], best_hex[2])
     if (not coward) or (not coward.valid) then return end
 
+    -- If 'attack_if_trapped' is set, the coward attacks the weakest unit it ends up next to
+    if cfg.attack_if_trapped then
+        local max_rating, best_target = -9e99
+        for xa,ya in H.adjacent_tiles(coward.x, coward.y) do
+            local target = wesnoth.get_unit(xa, ya)
+            if target and wesnoth.is_enemy(coward.side, target.side) then
+                local rating = - target.hitpoints
+
+                if (rating > max_rating) then
+                    max_rating, best_target = rating, target
+                end
+            end
+        end
+
+        if best_target then
+            AH.checked_attack(ai, coward, best_target)
+            if (not coward) or (not coward.valid) then return end
+        end
+    end
+
     AH.checked_stopunit_all(ai, coward)
 end
 

data/ai/micro_ais/micro_ai_wml_tag.lua

@@ -234,7 +234,7 @@ function wesnoth.wml_actions.micro_ai(cfg)
             H.wml_error("Coward [micro_ai] tag requires either id= key or [filter] tag")
         end
         required_keys = { "distance" }
-        optional_keys = { "id", "filter", "filter_second", "seek_x", "seek_y","avoid_x","avoid_y" }
+        optional_keys = { "attack_if_trapped", "id", "filter", "filter_second", "seek_x", "seek_y","avoid_x","avoid_y" }
         CA_parms = {
             ai_id = 'mai_coward',
             { ca_id = 'move', location = CA_path .. 'ca_coward.lua', score = cfg.ca_score or 300000 }