Properly cancel the rpc futures as to stop the client&server in the json-rpc tests

[DavyLandman]

This fixes the

Task java.util.concurrent.FutureTask@55362a0e[Not completed, task = java.util.concurrent.Executors$RunnableAdapter@140b2035[Wrapped task = org.eclipse.lsp4j.jsonrpc.json.ConcurrentMessageProcessor@1034a66c]] rejected from java.util.concurrent.ThreadPoolExecutor@296ea999[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 1]

prints that every test had.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46%. Comparing base (f6d6bbe) to head (a7bad44).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##              main   #2768   +/-   ##
=======================================
- Coverage       46%     46%   -1%     
- Complexity    6725    6727    +2     
=======================================
  Files          794     794           
  Lines        65923   65923           
  Branches      9888    9888           
=======================================
- Hits         30837   30835    -2     
- Misses       32696   32700    +4     
+ Partials      2390    2388    -2     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud