Merge bitcoin/bitcoin#31251: test: report detailed msg during utf8 response decoding error

a2c45ae test: report failure during utf8 response decoding (furszy)

Pull request description:

  Useful for debugging issues such bitcoin/bitcoin#31241 (comment).

  Prints the entire response content instead of printing only the position of the byte it can't be decoded.

  The diff between the error messages can be seen by running the `wallet_migration.py` functional test with the following patch applied:
  ```
  diff --git a/src/wallet/rpc/wallet.cpp b/src/wallet/rpc/wallet.cpp
  --- a/src/wallet/rpc/wallet.cpp(revision d65918c5da52c7d5035b4151dee9ffb2e94d4761)
  +++ b/src/wallet/rpc/wallet.cpp(date 1731005254673)
  @@ -801,7 +801,7 @@
               }

               UniValue r{UniValue::VOBJ};
  -            r.pushKV("wallet_name", res->wallet_name);
  +            r.pushKV("wallet_name", "\xc3\x28");
               if (res->watchonly_wallet) {
                   r.pushKV("watchonly_name", res->watchonly_wallet->GetName());
               }
  ```

ACKs for top commit:
  achow101:
    ACK a2c45ae
  theStack:
    re-ACK a2c45ae
  rkrux:
    tACK a2c45ae
  ismaelsadeeq:
    utACK a2c45ae

Tree-SHA512: 6abb524b5a215c51ec881eea91ebe8174140a88ff3874c8c88676157edae7818801356586a904dbb21b45053183315a6d71dbf917d753611d8e413776b57c484

Author: achow101

test/functional/test_framework/authproxy.py

@@ -188,7 +188,12 @@ def _get_response(self):
                 {'code': -342, 'message': 'non-JSON HTTP response with \'%i %s\' from server' % (http_response.status, http_response.reason)},
                 http_response.status)
 
-        responsedata = http_response.read().decode('utf8')
+        data = http_response.read()
+        try:
+            responsedata = data.decode('utf8')
+        except UnicodeDecodeError as e:
+            raise JSONRPCException({
+                'code': -342, 'message': f'Cannot decode response in utf8 format, content: {data}, exception: {e}'})
         response = json.loads(responsedata, parse_float=decimal.Decimal)
         elapsed = time.time() - req_start_time
         if "error" in response and response["error"] is None: