"This is beyond the scope of the course" is not a great answer without further clarification or commentary. As you say, the ethos of the university is that your instructor is someone whose qualifications and expertise lie far beyond the scope of any undergraduate course. The answer is justified for a student who asks a certain kind of question in class, because class time is limited and one must exercise judgment about what to say and cover in that limited time. Going off on a lengthy digression that is likely to be of interest to only one student and perhaps not even well understood by her is not a good use of class time. So I would expect an instructor to say "Come talk to me after class if you are interested in that."
If a student comes to talk to you in your office hours or your spare time, I think that she deserves some kind of answer. The answer may in fact be that the question lies beyond your expertise (and there is nothing inherently wrong with that; there is a lot of stuff out there...), but in that case you should still spend at least a little while trying to direct the student elsewhere, either to the appropriate reading materials or to some other faculty member who can better help them out.
If you do feel that you know the answer to the question -- or at least, enough of the answer to the question -- then, sure, take a crack at answering it. It takes a lot of expertise -- subject expertise, pedagogical expertise, and practice -- to be able to give answers to such questions which occupy a reasonable amount of time and are at least somewhat meaningful to the student. This may involve for instance asking some quick questions of your own, trying to understand the student's background and the true direction and depth of their interest. One mistake that even seasoned pros make is to open up the gates and flood the student with information of a quantity, density and sophistication that is beyond what they can be expected to process in the moment. If someone asks you about the example of a conservative vector field on the punctured plane which is not a gradient field that you discussed in class, you should probably not respond by giving them a half hour lecture on DeRham cohomology. (At least not at first. One of the amazing things about teaching is that the chance that a multivariable calculus student really is looking for a lecture on DeRham cohomology in answer to their question is very, very small...but it is positive!)
How should I handle students who are interested but asks question beyond the course in the sense it requires an additional course or two to truly appreciate its importance or at least to see how the actual calculations are performed.
You take a shot at it. Make your first shot very brief: just drop some terminology and try to give a sentence or two expressing one of the main ideas in broadest terms. In the above case, you might say "Whether every conservative field is a gradient field depends on the domain. We saw that this is the case when the vector field is defined on the entire plane [or all of three-dimensional space...]. It is also true if the domain is something like an open disk or ball. It turns out though that 'holes in the domain' lead to conservative vector fields which are not gradient fields." (By the way, I first wrote more and then deleted some of it! Restraint is truly hard.)
I could tell them the answer but sometimes it can lead a student down a rabbit hole which can be devastating given how busy first year students are.
I'm not really sure what you mean by this. The type of personality that is going to be "devastated" by learning that things go deeper than they currently know does not seem well suited to higher education. If anything I feel exactly the opposite way: as an educator at any level, showing students the rabbit hole is one of the most important things that you can do. Especially, getting an undergraduate degree is all about learning just enough to get an awareness of the true depth of knowledge and acquiring a sound foundation upon which more knowledge can be built.
Further, I don't want to disrupt their "natural course" by saying something that may prevent independent self discovery.
Again, I don't really buy into this. Students who want to be sufficiently well insulated from being taught things by other people do not belong in a university. Self discovery is a wonderful and important thing, but it is enriched and reinforced by prior knowledge and coursework, not ruined by it. There are plenty of things to discover for oneself, and anyway you are only telling them a little. It seems very likely that such a conversation would if anything trigger the student's independent learning and self discovery, not inhibit it.
Lastly, I don't want to say something which could be misconstrued as a test topic.
This is why it's best to address such questions outside of the classroom, or at least outside of the class session, and ideally with only the students who are explicitly interested. It should then be much clearer that what you are telling them is not a test topic. If there is any ambiguity about that then you should resolve it. For instance, maybe the question actually is closely related to a test topic but comes from a direction in which the student does not see that. In that case you should point out the connection to them.