feat: add log transform to Logs for test-time message rewriting

Logs::setTransform(fn) installs a function that transforms every log
message before output. Useful in tests to replace raw r-addresses
with human-readable account names.

Usage:
  env.app().logs().setTransform([&](std::string const& text) {
      std::string out = text;
      // replace rG1QQv2... with Account(alice)
      boost::algorithm::replace_all(out, toBase58(alice.id()), "Account(alice)");
      return out;
  });
  // Pass nullptr to clear:
  env.app().logs().setTransform(nullptr);

include/xrpl/basics/Log.h

@@ -27,6 +27,7 @@
 #include <fstream>
 #include <map>
 #include <memory>
+#include <functional>
 #include <mutex>
 #include <utility>
 
@@ -165,6 +166,7 @@ private:
     beast::severities::Severity thresh_;
     File file_;
     bool silent_ = false;
+    std::function<std::string(std::string const&)> transform_;
 
 public:
     Logs(beast::severities::Severity level);
@@ -203,6 +205,17 @@ public:
         std::string const& text,
         bool console);
 
+    /** Set a transform applied to every log message before output.
+     *  Useful in tests to replace raw account IDs with human-readable names.
+     *  Pass nullptr to clear.
+     */
+    void
+    setTransform(std::function<std::string(std::string const&)> fn)
+    {
+        std::lock_guard lock(mutex_);
+        transform_ = std::move(fn);
+    }
+
     std::string
     rotate();
 

src/libxrpl/basics/Log.cpp

@@ -196,9 +196,10 @@ Logs::write(
     std::string const& text,
     bool console)
 {
-    std::string s;
-    format(s, text, level, partition);
     std::lock_guard lock(mutex_);
+    std::string const& transformed = transform_ ? transform_(text) : text;
+    std::string s;
+    format(s, transformed, level, partition);
     file_.writeln(s);
     if (!silent_)
         std::cerr << s << '\n';