fix: add direct tool registration API

[mikehostetler]

Summary

• add struct-level register_tool_direct/3 and unregister_tool_direct/2 APIs
• keep ReAct tools, actions_by_name, and reqllm_tools synchronized without AgentServer signals
• cover direct registration validation and unregistration behavior

Verification

• mix test test/jido_ai/jido_ai_core_test.exs
• mix test

Notes

This is the tactical unblocker for actions already executing inside an agent, such as dynamic MCP sync. A first-class dynamic tool provider remains the cleaner longer-term direction.