Fix ADAD command and release 4.2.6

[tctco]

What changed

• update the Slicer AI decoupling launcher to call the registered adad subcommand instead of the removed decouple command
• add a regression test covering the generated ADAD invocation
• bump the core release version from 4.2.5 to 4.2.6 in CMake, Conan, and the committed generated version header

Root cause

The core CLI renamed the decoupling entry point to adad, but the Slicer frontend continued invoking decouple. Argparse therefore rejected the command before processing the image.

User impact

Abeta and Tau signal decoupling can launch successfully from Slicer with DCCCcore 4.2.6.

Validation

• UV_CACHE_DIR=/tmp/dcccslicer-uv-cache uv run --with pytest pytest localizer/src/tests/test_ui_ai_decoupling.py localizer/src/tests/test_ui_metric_command_builder.py -q (7 passed)
• audited all other frontend and Python-wrapper subcommand names against the currently registered DCCCcore command catalog
• git diff --check