[HS3] Align RooFit HS3 JSON I/O with the current HS3 naming and HistFactory conventions.

[Phmonski]

Summary

Align RooFit HS3 JSON I/O with the current HS3 naming and HistFactory conventions.

• Replace RooBinWidthFunction HS3 type binwidth with:
  • binvolume for divideByBinWidth == false
  • inverse_binvolume for divideByBinWidth == true
• Stop exporting divideByBinWidth and remove legacy binwidth import support.
• Extend generic expression cleanup for floor, ceil, abs, tan, asin, acos, atan, PI, and EULER.
• Treat PI and EULER as reserved expression constants on import.
• Migrate HistFactory modifier export from constraint_name to constraint, and omit constraint_type.
• Keep HistFactory import compatibility for constraint, constraint_name, and legacy constraint_type.
• Export const: true for parameters whose min >= max.
• Export both RooHistFunc and ParamHistFunc as step.
• Add a step importer dispatcher that selects RooHistFunc for data and ParamHistFunc for parameters.
• Keep legacy histogram import support for old RooHistFunc HS3 files.
• Rename the following exporter keys:
  • generic_function -> generic
  • gauss_model_function -> gauss_resolution_model
  • truth_model_function -> delta_resolution_model
  • mixture_model -> mixture_resolution_model
  • fft_conv_pdf -> fft_convolution_dist

Tests

Added focused coverage in testRooFitHS3.cxx for:

• binvolume / inverse_binvolume export and import
• fixed-range parameter export as const: true
• step export/import dispatch for RooHistFunc and ParamHistFunc
• HistFactory constraint export with legacy constraint_name import
• generic expression cleanup for new function names and constants

[x] Tested locally

[github-actions]

Test Results

    20 files      20 suites   3d 8h 37m 25s ⏱️
 3 876 tests  3 874 ✅ 0 💤  2 ❌
68 505 runs  68 474 ✅ 0 💤 31 ❌

For more details on these failures, see this check.

Results for commit bf36555.

[Phmonski]

This PR contains a bug that is investigated.