Merge #19004: refactor: Replace const char* to std::string

c57f03ce17 refactor: Replace const char* to std::string (Calvin Kim)

Pull request description:

  Rationale: Addresses #19000
  Some functions should be returning std::string instead of const char*.
  This commit changes that.

  Main benefits/reasoning:

  1.  The functions never return nullptr, so returning a string makes code at call sites easier to review (reviewers don't have to read the source code to verify that a nullptr is never returned)
  2. All call sites convert to string anyway

ACKs for top commit:
  MarcoFalke:
    re-ACK c57f03ce17 (no changes since previous review) 🚃
  Empact:
    Fair enough, Code Review ACK c57f03ce17
  practicalswift:
    ACK c57f03ce17 -- patch looks correct
  hebasto:
    re-ACK c57f03ce17

Tree-SHA512: 9ce99bb38fe399b54844315048204cafce0f27fd8f24cae357fa7ac6f5d8094d57bbf5f5c1f5878a65f2d35e4a3f95d527eb17f49250b690c591c0df86ca84fd

src/bitcoin-cli.cpp

@@ -21,6 +21,7 @@
 #include <functional>
 #include <memory>
 #include <stdio.h>
+#include <string>
 #include <tuple>
 
 #include <event2/buffer.h>
@@ -158,7 +159,7 @@ struct HTTPReply
     std::string body;
 };
 
-static const char *http_errorstring(int code)
+static std::string http_errorstring(int code)
 {
     switch(code) {
 #if LIBEVENT_VERSION_NUMBER >= 0x02010300