Fix effective_context variable scope and calculate total tokens for streaming responses

- Pass effective_context as parameter to stream_generator functions
- Update _create_streaming_response signature to accept effective_context
- Update all calls to _create_streaming_response to pass effective_context
- Track accumulated response text for token counting in streaming
- Calculate completion tokens for Google responses (since Google doesn't provide them)
- Calculate completion tokens for non-Google providers when they don't provide token counts
- Include prompt_tokens, completion_tokens, total_tokens, and effective_context in final chunk
- Fixes 'name effective_context is not defined' error in streaming responses
- Fixes issue where streaming responses had null token counts