### Summary:
Towards #767. We were caching the list of tools for an agent, so if you
did `agent.tools.append(...)` from a tool call, the next call to the
model wouldn't include the new tool. THis is a bug.
### Test Plan:
Unit tests. Note that now MCP tools are listed each time the agent runs
(users can still cache the `list_tools` however).
## Summary:
Towards #404. I made this configurable because it's not clear this is
always a good thing to do. I also made it default to False because I'm
not sure if this could cause errors.
If it works out well, we can switch the default in the future as a small
breaking changes
## Test Plan:
Unit tests
