make NewTools return *Tools for consistency with NewMessages

[x2d7]

Summary

NewMessages() returns *Messages, but NewTools() returned Tools by value — forcing callers to take the address manually with &toolList.
Since both types contain a mutex internally, returning by pointer is also the correct choice to prevent accidental copies.

Changes

• NewTools() now returns *Tools instead of Tools
• All test setup updated to remove manual & dereference

// before
toolList := tools.NewTools()
c := chat.Chat{Tools: &toolList}

// after
c := chat.Chat{Tools: tools.NewTools()}