✨ envtest: respect pre-configured binary paths in ControlPlane

[mzhaom]

This commit fixes an issue where Environment.Start() would ignore pre-configured binary paths (APIServer.Path, Etcd.Path, KubectlPath) set in ControlPlane when DownloadBinaryAssets is false.

Changes:

• Extract path configuration logic into configureBinaryPaths() method for better testability and separation of concerns
• Only auto-configure binary paths when they are empty (not pre-set)
• When DownloadBinaryAssets is true, downloaded paths are still used (preserving existing behavior)
• Update ControlPlane field documentation to clarify path behavior
• Add tests in envtest_test.go to verify path handling logic

This fixed issue #3373

[k8s-ci-robot]

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[alvaroaleman]

/ok-to-test