fix issue where probuilder with quad topology instead of triangle would trigger out of bounds exception when being exported

[varinotmUnity]

Purpose of this PR

The degenerate triangle removal code introduced in v6.0.6 assumed all submeshes used triangular topology,
but ProBuilder meshes can contain mixed topologies (triangles, quads, etc.). The code incremented through indices
by 3 and accessed indexes[tri + 2], causing IndexOutOfRangeException when processing non-triangular submeshes
like quads with indices arrays not divisible by 3. This specifically affected shapes like Door that use quad topology.

Fix: Added a topology check if (submeshes[i].m_Topology == MeshTopology.Triangles) before applying the
degenerate triangle removal logic. This ensures the triangle-specific degenerate detection only runs on actual
triangular submeshes, preventing the index bounds violation on quad/other topologies.

Preserves Original Fix: The degenerate triangle removal functionality remains completely intact for triangular
submeshes - it still detects and removes zero-area triangles using cross-product magnitude checks. Non-triangular
submeshes bypass this processing entirely, which is appropriate since the concept of "degenerate triangles" doesn't
apply to quads or other topologies. The fix maintains backward compatibility while eliminating the crash.

Result: Door shapes and other mixed-topology meshes can now be exported without IndexOutOfRangeException,
while triangular meshes continue to benefit from degenerate triangle cleanup and improved rendering quality.

Links

https://jira.unity3d.com/browse/PBLD-251

Comments to Reviewers

I did not check for degenerate quads in this PR, because I could not repro the issue the user was having with degenerate triangles (making the bloom flash weirdly)

[TimAidleyAtUnity]

Thank you for fixing the bug I introduced!

[codecov-github-com]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
Runtime/Core/ProBuilderMeshFunction.cs	94.11%	1 Missing ⚠️

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##           master     #619   +/-   ##
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  Coverage   33.86%   33.86%           
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  Files         277      277           
  Lines       34797    34802    +5     
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+ Hits        11783    11787    +4     
- Misses      23014    23015    +1     

Flag	Coverage Δ
mac_trunk	33.65% <94.11%> (+<0.01%)	⬆️
win_trunk	33.54% <94.11%> (+<0.01%)	⬆️

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

Files with missing lines	Coverage Δ
Runtime/Core/ProBuilderMeshFunction.cs	71.29% <94.11%> (+0.08%)	⬆️

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[TimAidleyAtUnity]

Nitpick: couldn't you merge both of these checks in to one?

if (submeshes[i].m_Topology == MeshTopology.Triangles && indexes.Length % 3 == 0)

[zatzuri-unity]

Tested the repro steps for this issue as well as the original repro steps from the degenerate triangles issues. I've also tested exporting different meshes