Google's native Gemini API uses the URL action :streamGenerateContent
to indicate streaming intent, not just the ?alt=sse query parameter.
The current IsStream() only checks c.Query("alt") == "sse", causing
all :streamGenerateContent requests (without ?alt=sse) to be treated
as non-streaming.
This adds a strings.Contains check for "streamGenerateContent" in the
request URL path, so both streaming indicators are recognized.
