Make `ThreeQubitSquash` work with circuits containing (some) boxes (#… #6421

Job	Run time
Check tket library version	15s
Build and test pytket (macos) (macos-13)	7m 41s
Build and test pytket (ubuntu)	13m 26s
Build and test pytket (windows)	11m 57s
Build and test pytket (macos) (macos-15)	9m 54s
Build and test (tket) (ubuntu-24.04)	32m 4s
Build and test (tket) (macos-13)	33m 0s
Build and test (tket) (windows-2025)	57m 46s
Build and test (tket) (macos-15)	14m 54s
Check documentation build	1m 1s
Check C++ code formatting	28s
Check pytket line and branch coverage	0s
Publish pytket coverage	17s
All build and test jobs successful (or skipped)	1s
	3h 2m 44s

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