Multiple class definitions in the same file raise error "invalid syntax"

Hey, there. It seems like it's not possible to define multiple classes in a single Python file with `py.RunSrc()`, unless I'm misunderstanding something...? It works with `py.RunFile()`, though.

Go:
```go

context := py.NewContext(py.DefaultContextOpts())

data, _ := fs.ReadFile(g.FS, "math_stuff.py")

_, err := py.RunSrc(context, string(data), "run", nil)

log.Println(err)

```

`math_stuff.py`
```python
class A:
pass

class B:
pass
```

Gives me the error:

```
2026/07/02 15:06:16
File "run", line 5, offset 5
class B:

SyntaxError: 'invalid syntax'
```

I'm testing with the latest commit.

Start by reproducing the issue with py.RunSrc and the two-class math_stuff.py example, then compare its handling with py.RunFile. Trace the parser or source-loading path reached by py.RunSrc and add a regression test for multiple class definitions; done means the example runs without the invalid syntax error.

索引モデルが issue の本文から書いたものです。