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.

Start in client.py and inspect how PodmanClient handles the connection and active_service paths when building api_kwargs. Reproduce the three calls from the issue, then verify that explicit identity is honored and that connections without an identity omit the argument without raising.

Written by the indexing model from the issue text.