REST_Geant4_MergeRestG4Files does not fully preserve/validate metadata and analysis outputs

[cmargalejo]

The Geant4 merge macro currently does not fully preserve or validate all metadata needed in merged restG4 ROOT files.

Update: PR #153 addresses the metadata-preservation and metadata-validation part of this issue. The AnalysisTree merge behavior remains a separate design/policy question.

Related prior work:

• geant4lib Merge supports analysis files #114: adds support for merging analysis files, but is stale/conflicting and only partially addresses metadata handling.
• restG4 Add validation for merge files restG4#101: adds a limited merge validation test for older merge work, but does not validate metadata compatibility or analysis-tree correctness.

Observed / diagnosed problems:

• TRestGeant4Metadata copy/merge behavior can drop particle-source information.
• TRestGeant4PhysicsLists is not robustly validated/preserved by the current merge path.
• Input files are assumed compatible, but compatibility is not checked.
• AnalysisTree handling differs between the current macro and Merge supports analysis files #114; Merge supports analysis files #114 preserves trees via TFileMerger but allows duplicate event IDs.
• Output validation only checks limited entry-count conditions and does not verify required metadata/classes.
• ROOT StreamerInfo loss in UPDATE-mode writes is related, but tracked separately in framework #568 / PR #567.

Expected fix:

• Preserve required metadata objects, including Geant4 metadata and physics-list metadata.
• Validate input compatibility before merging.
• Define and test the intended policy for AnalysisTree merging and duplicate event IDs.
• Validate merged output contains expected trees, geometry, metadata, and entry counts.
• Add restG4 integration coverage for simulation-only and analysis-file merge cases.

The merge should also validate that all input files were produced with compatible simulation metadata:

• same TRestGeant4PhysicsLists;
• same Geant4 version from TRestGeant4Metadata::GetGeant4Version();
• compatible generator/source, geometry, and storage settings.

[cmargalejo]

Update: #153 addresses the metadata-preservation and validation part of this issue.

That PR focuses on making REST_Geant4_MergeRestG4Files.C safer for restG4 simulation outputs by:

• preserving TRestGeant4PhysicsLists;
• validating Geant4 version and physics-list compatibility across inputs;
• deep-copying particle-source metadata;
• merging TRestGeant4PhysicsInfo lookup metadata with conflict checks;
• adding unit coverage for metadata copy/merge behavior.

This does not yet resolve the AnalysisTree merge behavior discussed in #114. That part should probably remain a
separate design decision, since TRestDataSet is intended to combine AnalysisTree data and may be the preferred
workflow.