fix: responses from /chat/completions endpoint contain a leading space in the content

[vansangpfiev]

Issue link: janhq/jan#2548

• Add google test with sample
• Trim the leading space if not in stream mode or it is the first token in stream mode

[vansangpfiev]

Nitro logs:
Call:

POST http://127.0.0.1:3928/v1/chat/completions
**headers**
content-type: application/json
**body**
{
    {
      "content": "Hello!",
      "role": "user"
    }
  ],
  "model": "mistral-7b-instruct-v0.1.Q5_0",
  "stream": true,
  "max_tokens": 4096,
  "stop": [
    "</s>"
  ],
  "frequency_penalty": 0,
  "presence_penalty": 0,
  "temperature": 0.7,
  "top_p": 0.95
}

Response:

data: {"choices":[{"delta":{"content":"I"},"finish_reason":null,"index":0}],"created":1712803949,"id":"KqHSWp7mbWJpCveUZhve","model":"_","object":"chat.completion.chunk"}
data: {"choices":[{"delta":{"content":"'"},"finish_reason":null,"index":0}],"created":1712803950,"id":"kmChXtpgvhb8AEVIIDQ1","model":"_","object":"chat.completion.chunk"}
data: {"choices":[{"delta":{"content":"m"},"finish_reason":null,"index":0}],"created":1712803951,"id":"bA3EIN56gaTXLLoFnLpv","model":"_","object":"chat.completion.chunk"}
data: {"choices":[{"delta":{"content":" just"},"finish_reason":null,"index":0}],"created":1712803951,"id":"uQ5lSv9u2E4GUCdUejk7","model":"_","object":"chat.completion.chunk"}
data: {"choices":[{"delta":{"content":" a"},"finish_reason":null,"index":0}],"created":1712803952,"id":"WhCvljFp5UFk1qjDgwmZ","model":"_","object":"chat.completion.chunk"}
...