fboss2 bgp config: address clang-tidy findings

- Use auto for the SimpleJSONSerializer template-cast result in
  ConfigSession::saveBgpConfig() (modernize-use-auto).
- Drop redundant .c_str() on the TEnumTraits::findValue() argument in
  the neighbor enum attribute handler (readability-redundant-string-cstr).

Author: hillol-nexthop

fboss/cli/fboss2/commands/config/protocol/bgp/neighbor/CmdConfigProtocolBgpNeighbor.cpp

@@ -233,8 +233,7 @@ AttrHandler enumAttr(
       mode = static_cast<EnumT>(*enable ? 1 : 0);
       valid = true;
     } else {
-      valid = apache::thrift::TEnumTraits<EnumT>::findValue(
-          values[0].c_str(), &mode);
+      valid = apache::thrift::TEnumTraits<EnumT>::findValue(values[0], &mode);
     }
     if (!valid) {
       std::string names;

fboss/cli/fboss2/session/ConfigSession.cpp

@@ -592,7 +592,7 @@ void ConfigSession::saveBgpConfig() {
 
   // Serialize the entire typed config (round-tripped through parse so integer
   // map keys become string keys, mirroring saveConfig() for the agent).
-  std::string json =
+  auto json =
       apache::thrift::SimpleJSONSerializer::serialize<std::string>(bgpConfig_);
   std::string prettyJson = folly::toPrettyJson(folly::parseJson(json));
   folly::writeFileAtomic(