This pull request resolves openai#777; If you think we should introduce a new
item type for MCP call output, please let me know. As other hosted tools
use this event, I believe using the same should be good to go tho.

)

The EmbeddedResource from MCP tool call contains a field with type
AnyUrl that is not JSON-serializable. To avoid this exception, use
item.model_dump(mode="json") to ensure a JSON-serializable return value.

### Summary:
Towards openai#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).

Closes openai#796. Shouldn't start a busy waiting thread if there aren't any
traces.

Test plan
```
import threading
assert threading.active_count() == 1
import agents
assert threading.active_count() == 1
```

### Summary:
Allows a user to do `function_tool(is_enabled=<some_callable>)`; the
callable is called when the agent runs.

This allows you to dynamically enable/disable a tool based on the
context/env.

The meta-goal is to allow `Agent` to be effectively immutable. That
enables some nice things down the line, and this allows you to
dynamically modify the tools list without mutating the agent.

### Test Plan:
Unit tests

bump version

## Summary
- describe semantic versioning and release steps
- add release page to documentation nav

## Testing
- `make format`
- `make lint`
- `make mypy`
- `make tests`
- `make build-docs`


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https://chatgpt.com/codex/tasks/task_i_68409d25afdc83218ad362d10c8a80a1