Trigger transmission helper from KHR_materials_diffuse_transmission.

[MiiBond]

This PR separates out the transmissionHelper from the KHR_materials_transmission and instantiates it from KHR_materials_diffuse_transmission as well.

[Popov72]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
refs/pull/18451/merge

Test environment:
https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/18451/merge/index.html

To test a playground add it to the URL, for example:

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/18451/merge/index.html#WGZLGJ#4600

Links to test your changes to core in the published versions of the Babylon tools (does not contain changes you made to the tools themselves):

https://playground.babylonjs.com/?snapshot=refs/pull/18451/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/18451/merge
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If you made changes to the sandbox or playground in this PR, additional comments will be generated soon containing links to the dev versions of those tools.

[bjsplat]

Interaction tests

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/18451/merge/testResults/interactionplaywright/index.html

[bjsplat]

Visualization tests for WebGPU

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/18451/merge/testResults/webgpuplaywright/index.html

[bjsplat]

⚡ Performance Test Results

🟢 All performance tests passed — no regressions detected.

[bjsplat]

🟢 Memory Leak Test Results

13 passed, 0 leaked out of 13 scenarios

🟢 All memory leak tests passed — no leaks detected.

Passed Scenarios (13)

Scenario	Package
Core Feature Stack	@babylonjs/core
Core Rendering Materials Shadows Stack	@babylonjs/core
Core Textures Render Targets PostProcess Stack	@babylonjs/core
GUI Fullscreen UI Controls	@babylonjs/gui
GUI Mesh ADT Controls	@babylonjs/gui
Loaders Boombox Import	@babylonjs/loaders
Loaders OBJ Direct Load	@babylonjs/loaders
Loaders STL Direct Load	@babylonjs/loaders
Materials Library Stack	@babylonjs/materials
Serializers glTF Export	@babylonjs/serializers
Serializers GLB Export	@babylonjs/serializers
PostProcesses Digital Rain Stack	@babylonjs/post-processes
Procedural Textures Stack	@babylonjs/procedural-textures

[bjsplat]

WebGL2 visualization test reporter:

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/18451/merge/testResults/webgl2playwright/index.html