Know its too late know, however next time dry pressure test any new pipework before filling with water.
Empty and thoroughly dry the section of pipework, vent if you can, try warming with a hair dryer, once dry add some flux (lead free) and re-solder, pressure test and if you still haven’t sealed it cut it out and start again.
Could just go to cut it out, but its always worth a try.
You can re-solder joints as long as you get all the water out of the pipe/fitting.
The easiest way is to prevent it - make sure the pipe and fitting are clean - use wirewool to clean all pipe ends and inside fitting. Apply flux to fitting (apply to pipe if for gas) - make sure there's no water in the pipes.
Do that everytime - they very very rarely fail - no need to pressure test.
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