fix: address SonarCloud reliability issues

[tomdesair]

This PR addresses three reliability issues identified by SonarCloud:

1. Infinite Recursion in RequestHandler: The default process(5 args) method called the process(7 args) method, which in turn defaulted to calling the process(5 args) method. If neither was implemented, this would result in a StackOverflowError. The 5 args method was modified to throw an UnsupportedOperationException to break the cycle and enforce the use of the newer 7 args method.
2. NullPointerException in RufhUploadExistsValidator: Added null checks for request and uploadStorageService at the start of the validate method. Previously, if they were null, Utils.isCreationEndpoint would return false, leading to a NPE when request.getRequestURI() was invoked.
3. NullPointerException in DiskLockingService: Added a null check for lockPath in getStopPath. getPathInStorageDirectory can return null, which would cause an NPE when calling .resolveSibling() on it.

Tests and test coverage were updated to ensure everything passes and code changes are well-covered.

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PR created automatically by Jules for task 1854060035614280304 started by @tomdesair

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[coveralls]

coverage: 96.483% (-0.04%) from 96.519% — fix-sonarcloud-reliability-issues-1854060035614280304 into master