Spark Trigger.AvailableNow fails when the initial streaming snapshot is expired

### Apache Iceberg version

1.11.0 (latest release)

### Query engine

Spark

### Please describe the bug 🐞

When using an Iceberg table as a Spark Structured Streaming source with `Trigger.AvailableNow`, restarting the query can fail after `expire_snapshots` removes the snapshot referenced by the stream's initial offset.

The checkpoint may already contain newer committed offsets whose snapshots are still available, but `prepareForTriggerAvailableNow()` computes the target offset from `initialOffset`:

```java
lastOffsetForTriggerAvailableNow =
(StreamingOffset) latestOffset(initialOffset, ReadLimit.allAvailable());
```

`latestOffset()` eventually validates the snapshot referenced by `initialOffset`. If this old snapshot has been expired, the query fails with:

```text
Cannot load current offset at snapshot ,
the snapshot was expired or removed
```

Example:

```text
initial offset: S0 -> expired
latest committed offset: S30 -> available
current snapshot: S40 -> available
```

The stream still fails because `S0` is used by `prepareForTriggerAvailableNow()`.

### Expected behavior

Once the checkpoint has advanced, the original initial snapshot should not need to be retained indefinitely. `AvailableNow` should resume from the current checkpoint position without requiring the snapshot referenced only by the original `initialOffset`.

### Environment

* Spark 4.0
* Iceberg 1.11.0
* `Trigger.AvailableNow`
* Iceberg `readStream`
* Persistent checkpoint
* Regular `expire_snapshots`

### Willingness to contribute

- [x] I can contribute a fix for this bug independently
- [x] I would be willing to contribute a fix for this bug with guidance from the Iceberg community
- [ ] I cannot contribute a fix for this bug at this time

Start at prepareForTriggerAvailableNow() and follow latestOffset(initialOffset, ReadLimit.allAvailable()) through the Spark Structured Streaming source offset validation. Reproduce the failure with Trigger.AvailableNow, a persistent checkpoint, and expire_snapshots removing the initial snapshot. Done means a restarted stream uses the advanced checkpoint position without requiring that expired initial snapshot.

Written by the indexing model from the issue text.