Fix null pointer dereferences after dynamic_cast (rewritten)

[jminor]

Certain invalid OTIO object graphs may cause crashes due to code that assumes a dynamic_cast always returns a valid object. This PR introduces tests which demonstrate the issue and a fix to prevent the crash in those cases.

I made sure to write the tests first, ensure that they caused the crash (SEGFAULT) and then made the fix to each of the affected functions. I introduced a new ErrorStatus::CANNOT_CLONE_ITEM for this specific case. The code may also return ErrorStatus::TYPE_MISMATCH in a related edge case, but I was not able to create a scenario which triggers that case.

Note that the discovery of this issue was assisted by Claude Code Opus 4.6 and Opus 4.7, but all of the code in this PR was written by a human (me). This PR replaces #2017 which had LLM-generated code in it.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.26%. Comparing base (c51d995) to head (40478d6).

Files with missing lines	Patch %	Lines
src/opentimelineio/algo/editAlgorithm.cpp	0.00%	42 Missing ⚠️
src/opentimelineio/errorStatus.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2020      +/-   ##
==========================================
- Coverage   84.50%   84.26%   -0.25%     
==========================================
  Files         181      181              
  Lines       13223    13261      +38     
  Branches     1202     1218      +16     
==========================================
  Hits        11174    11174              
- Misses       1866     1904      +38     
  Partials      183      183              

Flag	Coverage Δ
py-unittests	84.26% <0.00%> (-0.25%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/opentimelineio/errorStatus.h	100.00% <ø> (ø)
src/opentimelineio/errorStatus.cpp	51.61% <0.00%> (-1.73%)	⬇️
src/opentimelineio/algo/editAlgorithm.cpp	0.00% <0.00%> (ø)

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[meshula]

This looks more rigorous than the initial fix.

[darbyjohnston]

Thanks, I think this is clearer about what is being tested than the previous PR. When we are happy with this one, try asking Claude to compare the two and see what it thinks about the changes.

[darbyjohnston]

I think this error status should be set in clone(), so it is set only once and not at each call site. Something like:

auto cloned_item = items.front()->clone(error_status);
if (!cloned_item || (error_status && is_error(error_status))
    return;

SerializableObject*
SerializableObject::clone(ErrorStatus* error_status) const
{
    ...
    SerializableObject*r = e._root.type() == typeid(SerializableObject::Retainer<>)
               ? std::any_cast<SerializableObject::Retainer<>&>(e._root)
                     .take_value()
               : nullptr;
    if (!r && error_status)
        *error_status = ErrorStatus::CANNOT_CLONE_ITEM;
    return r;
}