fix: Support both "data:" and "data: " formats for streaming responses

[CaroyalKnight]

If the streaming output starts with data: instead of data: , for example:

data:{"candidates":[{"content":{"role":"model","parts":[{"text":"Learn ...","thought":true}]}}],"usageMetadata":{"trafficType":"ON_DEMAND"}}

an error will occur.

Traceback (most recent call last):
  File ".venv/lib/python3.12/site-packages/google/genai/_api_client.py", line 495, in _load_json_from_response
    return json.loads(response)
           ^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/anaconda3/lib/python3.12/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/anaconda3/lib/python3.12/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/anaconda3/lib/python3.12/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "genai_request.py", line 29, in <module>
    for chunk in stream:
                 ^^^^^^
  File ".venv/lib/python3.12/site-packages/google/genai/models.py", line 6658, in generate_content_stream
    for chunk in response:
                 ^^^^^^^^
  File ".venv/lib/python3.12/site-packages/google/genai/models.py", line 5024, in _generate_content_stream
    for response in self._api_client.request_streamed(
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.12/site-packages/google/genai/_api_client.py", line 1629, in request_streamed
    for chunk in session_response.segments():
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.12/site-packages/google/genai/_api_client.py", line 308, in segments
    yield self._load_json_from_response(chunk)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.12/site-packages/google/genai/_api_client.py", line 497, in _load_json_from_response
    raise errors.UnknownApiResponseError(
google.genai.errors.UnknownApiResponseError: Failed to parse response as JSON. Raw response: data:{"candidates":[{"content":{"role":"model","parts":[{"text":"Learn ...","thought":true}]}}],"usageMetadata":{"trafficType":"ON_DEMAND"}}

Therefore, some changes have been made to support both data: and data: .

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