DynamoDB Enhanced - DurationAttributeConverter incorrectly converts negative durations with nanosecond offsets

### Describe the bug

When converting a negative `Duration` with a non-zero nanosecond offset, DynamoDB enhanced client's `DurationAttributeConverter.transformFrom` will return a value that does not match the input. Passing that returned value back into `DurationAttributeConverter.transformTo` will not return a `Duration` equal to the original value.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Expected Behavior

I expect that I will receive an equivalent value after converting a negative `Duration` with a non-zero nanosecond offset to and from an `AttributeValue` with `DurationAttributeConverter`.

For example, if I create a `DurationAttributeConverter` named `converter` and call `converter.transformFrom(Duration.ofMillis(-1))`, I'd expect to receive `AttributeValue.fromN("-0.001000000")`. Subsequently calling `converter.transformTo` on that result should return `Duration.ofMillis(-1)`.

### Current Behavior

Rather than receiving an equivalent value, I received a different value `Duration.parse("PT-1.999S")` from the `transformTo` call and a value of `AttributeValue.fromN("-1.999000000")` from the `transformFrom` call.

### Reproduction Steps

```java
DurationAttributeConverter converter = DurationAttributeConverter.create();
Duration original = Duration.ofMillis(-1);
// This assertion will fail
assert original.equals(converter.transformTo(converter.transformFrom(original)));
```

### Possible Solution

The most likely reason for this bug is the internal representation used by Java for negative durations. From the `java.time.Duration.getSeconds` Javadoc:
> A negative duration is expressed by the negative sign of the seconds part. A duration of -1 nanosecond is stored as -1 seconds plus 999,999,999 nanoseconds.

### Additional Information/Context

_No response_

### AWS Java SDK version used

2.41.32

### JDK version used

openjdk 25.0.3 2026-04-21 LTS OpenJDK Runtime Environment Corretto-25.0.3.9.1 (build 25.0.3+9-LTS) OpenJDK 64-Bit Server VM Corretto-25.0.3.9.1 (build 25.0.3+9-LTS, mixed mode, sharing)

### Operating System and version

Rocky Linux release 8.10 (Green Obsidian), but will occur on any OS

Inizia da DurationAttributeConverter e analizza transformFrom e transformTo usando la riproduzione con Duration.ofMillis(-1) riportata nell’issue. Aggiungi un test di regressione che copra le durate negative con offset di nanosecondi diversi da zero, quindi esegui i test pertinenti del converter e verifica che il round trip restituisca la Duration originale e l’AttributeValue previsto.

Scritto dal modello di indicizzazione a partire dal testo della issue.