rts: answer a snapshot request made on a connection that is gone

[marksvc]

fetchSnapshotByTimestamp called through optional chaining, so when the
handle no longer named a connection the call was skipped entirely and
the
callback was never invoked. The dotnet caller awaits that callback
until it is given up on.

Check for the missing connection and answer with an error, as the
neighbouring fetchSnapshotsByTimestamp already does.

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Open in Devin Review

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.10%. Comparing base (dab7cf0) to head (9ef2fff).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4062   +/-   ##
=======================================
  Coverage   81.10%   81.10%           
=======================================
  Files         661      661           
  Lines       43060    43060           
  Branches     7033     7033           
=======================================
  Hits        34925    34925           
  Misses       6977     6977           
  Partials     1158     1158           

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