[Agent Engine] support SSE response

**Is your feature request related to a problem? Please describe.**
Currently, Agent Engine `https://LOCATION-aiplatform.googleapis.com/v1/projects/PROJECT_ID/locations/LOCATION/reasoningEngines/RESOURCE_ID:streamQuery?alt=sse` does not respond SSE response.
it is stream response, but content type is `application/json`.

**Describe the solution you'd like**
Support SSE response.

Start with the Agent Engine streamQuery endpoint shown in the issue and trace how the Python SDK handles its current application/json stream response. Done means the endpoint is handled as an SSE response with the expected content type, with the behavior verified by appropriate tests.

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