Fix how stream flag is read from request

[mreso]

This PR fixes how the stream flag is read from a request. Sending a request like:

curl http://localhost:5000/v1/chat/completions   -H "Content-Type: application/json"   -d '{
    "model": "llama3.1",
    "stream": "False",
    "max_tokens": 200,
    "messages": [
      {
        "role": "system",
        "content": "You are a helpful assistant."
      },
      {
        "role": "user",
        "content": "Hello!"
      }
    ]
  }'

currently results in a streaming response as a non-empty string (like 'False') is interpreted by bool() as True.

[pytorch-bot]

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❌ 1 New Failure, 1 Pending, 1 Unrelated Failure

As of commit 6065e37 with merge base 68b8087 ():

NEW FAILURE - The following job has failed:

• Run the aoti runner with CUDA using stories / test-runner-aot-cuda / linux-job (gh)
  RuntimeError: Command docker exec -t 87bf4783ee60c6bb6b0a8a777a960b996781a6788a6cf4cca412214db60ba011 /exec failed with exit code 1

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / runner-aoti (16-core-ubuntu) (gh) (trunk failure)
  Process completed with exit code 1.

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[Jack-Khuu]

Thanks