HttpTransferCache collapses distinct binary POST bodies into the same cache key

### Which @angular/* package(s) are the source of the bug?

common / http

### Is this a regression?

No

### Description

With POST requests enabled in `HttpTransferCache`, two requests with different binary bodies can end up sharing one cache entry. This matters for valid POST-based APIs such as Protobuf and gRPC, where the request body carries the input. The same cache-key rule also applies to other POST APIs, including GraphQL requests.

I reproduced this with two read-only Protobuf requests to the same URL. The bodies are valid, but different:

```text
document 100: 0864 document 200: 08c801
```

Angular should treat these as separate requests and cache their responses separately. Instead, the request for document `200` gets the cached response for document `100`; it never reaches the origin.

```ts
else if (typeof serializedBody !== 'string') {
serializedBody = '';
}
```

`HttpRequest.serializeBody()` returns a `Blob` as-is. Because a `Blob` is not a string, `makeCacheKey()` turns it into `''`. That means both Protobuf payloads have the same cache-key input before hashing.

### Please provide a link to a minimal reproduction of the bug
See Repository: https://github.com/SkyZeroZx/angular-protobuf-transfercache-poc

先在 common/http 中跟踪 makeCacheKey 和 HttpRequest.serializeBody()，然后使用链接的 angular-protobuf-transfercache-poc 重现该问题。当不同的二进制 POST body 产生不同的缓存键，且它们的响应不会被共享时，即视为完成。

由索引模型根据 Issue 内容生成。