PodmanClient(connection=...) unusable without an identity; explicit identity= is ignored

### Summary

`PodmanClient(connection=...)` cannot be used with a connection that has no identity, and passing an explicit `identity=` does **not** work around it, because the fallback is evaluated eagerly.

### Code (current `main`, `client.py`)

```python
api_kwargs["identity"] = kwargs.get("identity", str(connection.identity))
```

`str(connection.identity)` is a default *argument*, so Python evaluates it before `kwargs.get` runs — even when the caller supplied `identity`. With no identity configured, `connection.identity` is `Path(None)` and raises (see the separate report on `ServiceConnection.identity`).

The same pattern is used on the `active_service` path, so a bare `PodmanClient()` fails on a system whose active connection has no identity.

### Reproducer

```python
from podman import PodmanClient

PodmanClient(connection="remote-with-key") # OK
PodmanClient(connection="local") # TypeError
PodmanClient(connection="local", identity="/dev/null") # TypeError <-- explicit identity ignored
```

Measured against a host with a `unix://` connection named `local`.

### Expected

The explicit `identity=` is honoured, and a connection without one simply omits the argument.

### Suggested fix

Compute the fallback lazily:

```python
identity = kwargs.get("identity") or connection.attrs.get("Identity") or connection.attrs.get("identity")
if identity:
api_kwargs["identity"] = identity
```

### Environment

podman-py 5.6.0 (Fedora `python3-podman-5.6.0-4.fc44`), podman 5.8.4 client and server, rootless, Python 3.14. Re-checked against current `main` before filing.

client.py から始め、api_kwargs の構築時に PodmanClient が connection と active_service のパスをどのように処理するかを調べます。issue にある 3 つの呼び出しを再現し、そのうえで明示的な identity が尊重され、identity のない接続では例外を発生させずに引数が省略されることを確認します。

索引モデルが issue の本文から書いたものです。