Fix Random Ray Solver Bug: Detect Multiple Point Sources in Same Subdivided Source Region

[Copilot]

Problem

The Random Ray Solver had a silent bug where multiple point sources placed in the same subdivided source region would overwrite each other, with only the source having the higher index in the model::external_sources array being used. This could lead to incorrect simulation results without any warning to the user.

Root Cause

In FlatSourceDomain::convert_external_sources(), when mesh subdivision is enabled, point sources are mapped to a SourceRegionKey in the point_source_map_. The problematic line:

point_source_map_[key] = es;

This overwrites any existing entry for the same key, causing the silent loss of earlier point sources.

Solution

Added detection logic to identify when multiple point sources would map to the same SourceRegionKey and throw a clear error message instead of silently overwriting:

SourceRegionKey key {sr, mesh_bin};
auto it = point_source_map_.find(key);
if (it != point_source_map_.end()) {
  fatal_error("Multiple point sources detected in the same subdivided "
              "source region. This is not supported in the random ray "
              "solver.");
}
point_source_map_[key] = es;

Testing

The fix has been thoroughly tested with multiple scenarios:

• ✅ Error Case: Multiple point sources at the same location correctly trigger the error
• ✅ Valid Cases: Point sources at different locations continue to work normally
• ✅ Non-Mesh Cases: Multiple point sources work correctly when mesh subdivision is disabled
• ✅ Regression: All existing unit tests continue to pass

Benefits

• Prevents silent data corruption and incorrect simulation results
• Provides users with a clear, actionable error message
• Maintains full backward compatibility for legitimate use cases
• Follows the issue's guidance to provide an error rather than attempt complex feature support

The fix is minimal (5 lines of code) and surgical, addressing exactly the issue described while preserving all existing functionality.

Fixes #3470.

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[jtramm]

Looks good to me. Maybe change the wording slightly but otherwise this looks good.

[yrrepy]

Curious about the Black Arts,
did Copilot do this on its own, or did you have to ask (steer) it to do so?

[shimwell]

@Copilot it looks like the formatting of the C++ files is failing the CI checks

[jon-proximafusion]

@Copilot it looks like the formatting of the C++ files is failing the CI checks, can you correct the formatting of the C++ files you changed

[Copilot]

@copilot it looks like the formatting of the C++ files is failing the CI checks

Fixed the C++ formatting issue. The multi-line string in the error message now properly conforms to the 80-character column limit as required by the project's clang-format configuration. Changes made in commit 7211442.

[jtramm]

@yrrepy - I had opened a github issue for this item as I was planning to fix it slightly later but didn't want to forget. I noticed that when creating the issue, it allowed me to "assign" the issue to copilot instead of a human user, so figured I'd give it a try. It looks like it will only respond to comments from the person who assigned it the task.

[jtramm]

This looks good to me. However, I think a good policy for agent generated PR's is to have the formal reviewer still be someone separate from the one who assigned the task and worked with the agent to make the PR.

[shimwell]

I would be tempted to add a test that shows this fatal_error is raised appropriately.