[dolmatovDan]

#634 broke ci-python on main (run https://github.com/UnitTestBot/usvm/actions/runs/41299659885): with cpythonActivated=true, Gradle fails to create :usvm-python:buildSamples with Extension of type 'SourceSetContainer' does not exist. In registerBuildSamplesTask, the replacement for the sourceSets accessor calls extensions inside the tasks.register<JavaExec> { ... } configuration block, where it resolves to the task's extensions instead of the project's. The removed accessor was defined only on Project, so it did not have this problem.

Use project.extensions there. The other replacements from #634 are called directly on Project and are unaffected.

Validation: with -PcpythonActivated=true, both ./gradlew -m :usvm-python:test and ./gradlew tasks --all (which creates every task in every project) fail on current main with the CI error and pass with this change. The Python tests themselves were not run locally, since they require a CPython build; my validation for #634 missed this because it ran without cpythonActivated, so the lazily registered task was never created.

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