Propagate context variables in the SSE transport within the MCP integration

Related: [https://github.com/getsentry/sentry-python/pull/6583#discussion_r3427253810]()

See `test_sse_transport_detection_v2` test in `test_mcp.py`as a reference to this behaviour.

Because we don't propagate these correctly, traces lose everything that's set on the isolation and current scopes, we lose the parent-child span relationship, attributes that were set are lost, andwe get two traces instead of one.

**Note:** Rather than working to propagate the context variables, we may want to consider dropping support for the SSE transport as current data points to most users using the HTTP transport.