Live API: fractional-second Durations ("7.280s") fail to deserialize, dropping the whole message

### Description

`CustomDurationDeserializer` in `JsonSerializable` only parses **whole-second** `Duration` values (`Long.parseLong`), but the Gemini **Live API** server streams **fractional-second** Durations (e.g. `"7.280s"`, `"1.600s"`, `"0.360s"`). The parse throws `NumberFormatException`, which surfaces as:

```
SEVERE: Error deserializing message
com.google.genai.errors.GenAiIOException: Failed to deserialize the JSON node.
Caused by: com.fasterxml.jackson.databind.exc.InvalidFormatException:
Cannot deserialize value of type `java.time.Duration` from String "7.280s":
Cannot parse duration from string: 7.280s. Expected format 'Xs'.
```

Because this happens inside the Live WebSocket read path, the **entire server message is dropped**. When that message carries the model turn / a function call, the turn is lost — in a voice-agent app the assistant cuts off mid-sentence and no tool call fires.

### Impact

Production voice-reception app on `gemini-3.1-flash-live-preview` (Vertex): **~700 of these errors in 24h across ~190 calls** (~3–4 per call). Most calls survive because the read loop resumes on the next message, but an intermittent few break at the worst moment (mid first response / on the transfer tool call).

### Root cause

`src/main/java/com/google/genai/JsonSerializable.java`, `CustomDurationDeserializer`:

```java
long seconds = Long.parseLong(secondsPart); // rejects "7.280"
return java.time.Duration.ofSeconds(seconds);
```

### Suggested fix

Parse decimal seconds and keep sub-second precision:

```java
double seconds = Double.parseDouble(secondsPart);
return java.time.Duration.ofNanos(Math.round(seconds * 1_000_000_000.0));
```

(Protobuf `google.protobuf.Duration` is routinely JSON-encoded with fractional seconds, so this is the expected wire format.)

### Versions

- google-genai **1.61.0** (also present in 1.62.0 — `JsonSerializable` unchanged)
- Model: `gemini-3.1-flash-live-preview`, Vertex AI, Live API (BidiGenerateContent)
- Java 25