Fix critical error-hiding fallbacks in AbstractMCTSAI

Two issues fixed in pre-existing code:

1. Line 35: No tree built - Now properly distinguishes between:
   - null root (throws MCTSInternalError)
   - 0 actions available (throws - cannot return valid index)
   - 1 action (returns index 0, legitimate early exit)
   - Multiple actions but no children (throws - BuildMCTSTree bug)

2. Line 404: Empty actions list - Now throws instead of returning 0
   (which would be an invalid index into an empty list)

Defensive fallbacks retained:
- FILTERED_RANDOM falls back to random from all actions (reasonable)
- BEST_IMMEDIATE falls back to first action (reasonable)

These fallbacks are acceptable defensive programming against overly
aggressive filtering and don't hide bugs.

src/main/cpp/net/eagle0/common/mcts/abstract/AbstractMCTSAI.cpp

@@ -31,11 +31,31 @@ auto AbstractMCTSAI::Search(
     result.searchTime = std::chrono::duration_cast<std::chrono::milliseconds>(
             std::chrono::steady_clock::now() - startTime);
 
-    if (!rootNode || rootNode->children.empty()) {
-        // Fallback to first action if no tree was built
-        result.bestActionIndex = 0;
-        result.bestScore = 0.0;
-        return result;
+    if (!rootNode) {
+        throw MCTSInternalError("MCTS search: BuildMCTSTree returned null root node");
+    }
+
+    if (rootNode->children.empty()) {
+        // This can happen legitimately when:
+        // 1. No legal actions available (terminal state)
+        // 2. Only one action and we early-exited without exploring
+        // In case 1, there's no valid action to return. In case 2, we should have
+        // expanded that one child. Check totalActions to distinguish.
+        if (rootNode->totalActions == 0) {
+            throw MCTSInternalError(
+                    "MCTS search: No legal actions available in initial state - cannot return an "
+                    "action index");
+        }
+        // Single action case - should have been expanded in BuildMCTSTree
+        if (rootNode->totalActions == 1) {
+            result.bestActionIndex = 0;
+            result.bestScore = 0.0;
+            return result;
+        }
+        // Multiple actions but no children expanded - this shouldn't happen
+        throw MCTSInternalError(
+                "MCTS search: Root has " + std::to_string(rootNode->totalActions) +
+                " actions but no children expanded - this indicates a bug in BuildMCTSTree");
     }
 
     // Find best child
@@ -381,7 +401,11 @@ auto AbstractMCTSAI::SelectSimulationAction(
         const MCTSGameState& state,
         const std::vector<std::unique_ptr<MCTSAction>>& actions,
         const bool isMaximizing) const -> size_t {
-    if (actions.empty()) { return 0; }
+    if (actions.empty()) {
+        throw MCTSInternalError(
+                "MCTSSimulation called with empty actions list - this indicates a bug in the "
+                "MCTS tree building or game state");
+    }
 
     thread_local std::mt19937 gen(std::random_device{}());