Convert plugin-webgpu-test-pipeline.yml to 1ES template
Mirrors the structure of plugin-webgpu-pipeline.yml by extending v1/1ES.Official.PipelineTemplate.yml@1esPipelines. Sets sdl.sourceAnalysisPool explicitly since there is no top-level pool; stage templates pin their own pools. Omits codeSignValidation since this pipeline does not produce or publish binaries.
