fix: Repair the Geant4 telescope detector geometry

[andiwand]

The Geant4 geometry did not match the tracking geometry:

• Of the two identically sized envelopes, the inner one was placed at the
  center of the layer stack and therefore protruded out of its mother,
  leaving Geant4 navigation undefined over most of the telescope. Both are
  replaced by a single envelope placed directly in the world.
• The silicon planes treated bounds as full lengths, while
  buildTelescopeDetector uses them as half lengths, so the Geant4 sensors
  were half the size of the ACTS surfaces in each direction.
• The world size assumed sorted and positive positions.

Also turns a segfault on a TelescopeDetector without positions into an
exception, and makes the Geant4 sources PRIVATE so that consumers of
ActsExamplesDetectorTelescope stop recompiling them.

--- END COMMIT MESSAGE ---

Noticed when running the Geant4 part of telescope_simulation.py.

Adds two tests:

• Tests/UnitTests/Examples/Detectors/TelescopeDetector checks the volume tree for overlaps and asserts that every sensitive Geant4 volume sits at an ACTS surface center with matching bounds, for all three telescope axes. It fails on the old geometry.
• test_telescope_geant4_geometry builds the Geant4 geometry from Python and maps it with SensitiveSurfaceMapper.

🤖 Generated with Claude Code

[benjaminhuth]

LGTM!