Address some AI review feedback: predeclare, include, format, comment

- Predeclare type reference in Rules.h
- Remove an unneeded include in EscrowToken_test
- Number_test will format negative BigInts correctly (unused)
- Remove an inaccurate comment

include/xrpl/protocol/Rules.h

@@ -123,6 +123,7 @@ private:
 };
 
 class NumberSO;
+class NumberMantissaScaleGuard;
 
 bool
 useRulesGuards(Rules const& rules);

src/test/app/EscrowToken_test.cpp

@@ -28,7 +28,6 @@
 #include <xrpl/protocol/Issue.h>
 #include <xrpl/protocol/MPTIssue.h>
 #include <xrpl/protocol/Protocol.h>
-#include <xrpl/protocol/Rules.h>
 #include <xrpl/protocol/SField.h>
 #include <xrpl/protocol/STAmount.h>
 #include <xrpl/protocol/TER.h>

src/test/basics/Number_test.cpp

@@ -31,7 +31,7 @@ class Number_test : public beast::unit_test::Suite
         int count = 0;
         for (auto it = s.rbegin(); it != s.rend(); ++it)
         {
-            if (count != 0 && count % 3 == 0)
+            if (count != 0 && count % 3 == 0 && isdigit(*it))
                 out.insert(out.begin(), '_');
             out.insert(out.begin(), *it);
             ++count;
@@ -1596,7 +1596,6 @@ public:
         constexpr std::int64_t kAValue = 1'000'000'000'000'049'863LL;
         constexpr std::int64_t kBValue = 9'223'372'036'854'315'903LL;
 
-        // Public conversion operator: STAmount::operator Number() const.
         Number const a = kAValue;
         Number const b = kBValue;
         Number const product = a * b;