[BUG] AI token limiter can miss usage from gzip responses split across buffers

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/shenyu/issues) and found no similar issues.

### Apache ShenYu Component

shenyu-plugin

### What happened

`AiTokenLimiterPlugin` records token usage by reading response chunks and parsing `completion_tokens`. For gzip responses it creates a raw `Inflater` and manually skips the gzip header only in the first buffer:

```java
final Inflater inflater = isGzip ? new Inflater(true) : null;
final AtomicBoolean headerSkipped = new AtomicBoolean(!isGzip);
...
if (headerSkipped.compareAndSet(false, true)) {
offset = skipGzipHeader(inBytes);
}
inflater.setInput(inBytes, offset, inBytes.length - offset);
```

`skipGzipHeader(...)` assumes the entire gzip header is available in that first buffer:

```java
private int skipGzipHeader(final byte[] b) {
int pos = 10;
int flg = b[3] & 0xFF;
...
return pos;
}
```

A gzip stream can be split across arbitrary `DataBuffer`s. If the first buffer is shorter than the minimum gzip header, or if optional gzip header fields are split across buffers, `skipGzipHeader(...)` throws or returns an offset based on incomplete data. The exception is caught by the outer `doOnNext` handler:

```java
} catch (Exception e) {
LOG.error("read dataBuffer error", e);
}
```

but `headerSkipped` has already been set to `true`, so later buffers are fed to the raw inflater as if the header had been consumed correctly. The decompressed text is then missing or corrupt, `completion_tokens` is not found, and `doFinally(...)` records `0` tokens from the empty/incomplete writer output.

This means gzip response chunking can cause AI token usage to be undercounted, allowing later requests that should have been limited.

### Expected behavior

The token limiter should handle gzip streams across buffer boundaries. It should either use a streaming gzip decoder, aggregate the bytes needed for decompression, or otherwise wait until the full gzip header is available before setting `headerSkipped` and feeding compressed bytes to the inflater.

### How to reproduce

1. Enable `ai-token-limiter` for an AI streaming endpoint.
2. Have the upstream return `Content-Encoding: gzip` and include usage data such as `"completion_tokens": 100`.
3. Make the gzip response arrive with the gzip header split across multiple `DataBuffer`s.
4. `skipGzipHeader(...)` fails or consumes an incomplete first chunk, later chunks cannot be decompressed correctly, and the plugin records `0` or incomplete token usage.

### Debug logs

_No response_

### Environment

Current `master` branch.

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

Start at AiTokenLimiterPlugin, especially skipGzipHeader(...) and the doOnNext handling that processes DataBuffers. Reproduce a gzip response whose header is split across buffers, then verify that completion_tokens is recovered and recorded instead of producing zero or incomplete usage.

Written by the indexing model from the issue text.