Move Wasm.Build.Tests into a dedicated WasmBuildTests pipeline stage

[Copilot]

Fixes #118938

Wasm.Build.Tests (WBT) jobs were inlined in the Build stage of every pipeline that runs them. This moves WBT into a dedicated WasmBuildTests stage across all such pipelines (runtime.yml, runtime-wasm.yml, runtime-wasm-non-libtests.yml), keeping the build-only producer jobs in Build.

Consumer templates

• browser-wasm-build-tests.yml and browser-wasm-coreclr-build-tests.yml gain a separateStage param (default false). When true, the cross-stage job-level dependsOn on the *_BuildOnly / *_CoreCLR producers is dropped — job dependencies are stage-scoped in AzDO. Producer artifacts are still pulled via the existing DownloadBuildArtifacts@0 (buildType: current), which resolves across stages.

runtime.yml

• Browser CoreCLR + browser Mono WBT relocated from Build into a new sibling - stage: WasmBuildTests.

Extra-platforms path

• New stages template extra-platforms/runtime-extra-platforms-wasm-build-tests.yml emits the WasmBuildTests stage (browser Mono + wasi), reusing the same compile-time guard so it never produces an empty stage.
• WBT consumers removed from the runtime-extra-platforms-wasm.yml jobs fragment; producers and wasm-runtime-tests stay in Build.
• runtime-wasm.yml and runtime-wasm-non-libtests.yml — the only two extra-platforms wrappers where WBT actually runs — include the new stage template.

Stage dependencies

The new stage depends on Build and EvaluatePaths (on PRs). WBT job conditions consume xplat-setup variables (shouldRunWasmBuildTestsOnDefaultPipeline, etc.) derived from $[ stageDependencies.EvaluatePaths… ]; without the EvaluatePaths dependency they would silently evaluate to empty.

- stage: WasmBuildTests
  displayName: Wasm.Build.Tests
  dependsOn:
  - Build
  - ${{ if eq(variables['Build.Reason'], 'PullRequest') }}:
    - EvaluatePaths

Note

Full AzDO template semantics (cross-stage artifact availability, stageDependencies resolution, skip propagation) can only be confirmed by running the pipelines on Azure DevOps. Local validation covered YAML parsing and stage topology.

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[pavelsavara]

This is actually slower