fix: handle Channel.DoesNotExist in remove_self

[ashnaaseth2325-oss]

SUMMARY

This PR fixes an unhandled Channel.DoesNotExist exception in the remove_self view that caused a 500 error for invalid channel_id. It ensures a proper 404 response is returned instead.
Primary changes are in contentcuration/contentcuration/viewsets/user.py.

---

FIX

• Code (before → after)

# Before
channel = Channel.objects.get(id=channel_id)

# After
try:
    channel = Channel.objects.get(id=channel_id)
except Channel.DoesNotExist:
    return HttpResponseNotFound(...)

---

RESULT

• Invalid channel_id now returns 404 Not Found instead of 500
• Prevents runtime crashes from uncaught exceptions
• Makes the existing error handling path reachable
• No impact on valid request flow

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