Update JAX backend handling for consistency across the codebase

[coderabbitai]

The addition of JAX to the DPBackend enum requires updates in multiple files to maintain consistency across the codebase:

• source/api_cc/src/DeepTensor.cc: Backend type checks need JAX handling
• source/api_cc/src/DeepPot.cc: Missing JAX case in backend comparisons
• source/api_cc/src/DataModifier.cc: Backend switch statements need JAX support

This issue was identified in PR #4307 (link) during the code review discussion here.

cc @njzjz