Calling Signal entity HTTP API fails with content-type=application/json

### Context
In the [http api documentation for signalling a durable entity](https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-http-api#signal-entity) it is stated that the request content (payload) should be JSON-formatted, implying that the Content-Type should be application/json.
Calling the API using this Content-Type=applicaton/json will throw an error in the get_input() function of the Durable entity context with the error "the JSON object must be str, bytes or bytearray, not dict"

### How to reproduce
Call the API using Content-Type = application/json

### Workaround
Call the API using Content-Type = text/plain

首先跟踪 Signal entity HTTP API 的处理流程，直到 Durable entity context 的 get_input() 函数，并使用已记录的 Signal entity 请求和报告的 Content-Type 行为作为复现用例。验证使用 application/json 时 JSON payload 能够被接受，同时保留现有的 text/plain workaround 行为；如果周边测试提供了合适的入口，则为这两种请求形式添加或更新覆盖测试。

由索引模型根据 Issue 内容生成。