Python: A2AAgent rejects int timeouts (only float/httpx.Timeout accepted)

### Motivation & Context

`A2AAgent(url=..., timeout=30)` raises `TypeError: Invalid timeout type: `. the type guard only accepts `float` and `httpx.Timeout`, but an int timeout is the most natural way to write it and httpx itself accepts ints everywhere (PEP 484 numeric tower: int is acceptable where float is expected).

### Reproduction

```python
from agent_framework.a2a import A2AAgent
A2AAgent(url="http://localhost:9999", timeout=30)
# TypeError: Invalid timeout type:
```

`timeout=30.0` works, `timeout=30` does not.

### Expected behavior

int timeouts coerce like floats.

env: python main

Start by locating the A2AAgent timeout type guard and inspect how it handles float and httpx.Timeout values. Reproduce the issue with timeout=30, then verify that integer timeouts are accepted and behave like float timeouts.

Written by the indexing model from the issue text.