args: make most ArgsManager members private

Move the first `protected` block (struct Arg, cs_args, m_settings, and
all other member variables) to `private`. Only `ReadConfigStream` and
`ReadConfigString` remain `protected` for test access.

Changes:
- Move `ReadConfigString` from `TestArgsManager` into `ArgsManager`
  itself (declared in args.h, defined in config.cpp) so tests no longer
  need direct access to `cs_args` or `m_settings` for config parsing.
- Replace test-only `SetNetworkOnlyArg` helper with the existing
  `NETWORK_ONLY` flag passed through `SetupArgs`/`AddArg`.
- Remove `TestArgsManager` constructor that cleared
  `m_network_only_args`.
- Remove `using` declarations for `cs_args`, `m_settings`, `GetSetting`,
  and `GetSettingsList` from `TestArgsManager`.
- Clear `m_config_sections` in `ClearArgs()`.

Co-authored-by: Anthony Towns <aj@erisian.com.au>

src/common/args.cpp

@@ -644,6 +644,7 @@ void ArgsManager::ClearArgs()
     m_settings = {};
     m_available_args.clear();
     m_network_only_args.clear();
+    m_config_sections.clear();
 }
 
 void ArgsManager::CheckMultipleCLIArgs() const

src/common/args.h

@@ -127,7 +127,7 @@ public:
         COMMAND = 0x800,
     };
 
-protected:
+private:
     struct Arg
     {
         std::string m_help_param;
@@ -149,8 +149,6 @@ protected:
     mutable fs::path m_cached_datadir_path GUARDED_BY(cs_args);
     mutable fs::path m_cached_network_datadir_path GUARDED_BY(cs_args);
 
-    [[nodiscard]] bool ReadConfigStream(std::istream& stream, const std::string& filepath, std::string& error, bool ignore_invalid_keys = false) EXCLUSIVE_LOCKS_REQUIRED(!cs_args);
-
     /**
      * Returns true if settings values from the default section should be used,
      * depending on the current network and whether the setting is
@@ -158,7 +156,11 @@ protected:
      */
     bool UseDefaultSection(const std::string& arg) const EXCLUSIVE_LOCKS_REQUIRED(cs_args);
 
- public:
+protected:
+    [[nodiscard]] bool ReadConfigStream(std::istream& stream, const std::string& filepath, std::string& error, bool ignore_invalid_keys = false) EXCLUSIVE_LOCKS_REQUIRED(!cs_args);
+    [[nodiscard]] bool ReadConfigString(const std::string& str_config) EXCLUSIVE_LOCKS_REQUIRED(!cs_args);
+
+public:
     /**
      * Get setting value.
      *

src/common/config.cpp

@@ -19,6 +19,7 @@
 #include <filesystem>
 #include <fstream>
 #include <iostream>
+#include <sstream>
 #include <list>
 #include <map>
 #include <memory>
@@ -119,6 +120,18 @@ bool ArgsManager::ReadConfigStream(std::istream& stream, const std::string& file
     return true;
 }
 
+bool ArgsManager::ReadConfigString(const std::string& str_config)
+{
+    std::istringstream streamConfig(str_config);
+    {
+        LOCK(cs_args);
+        m_settings.ro_config.clear();
+        m_config_sections.clear();
+    }
+    std::string error;
+    return ReadConfigStream(streamConfig, "", error);
+}
+
 bool ArgsManager::ReadConfigFiles(std::string& error, bool ignore_invalid_keys)
 {
     {