Skip code coverage tasks for docs-only PRs (#12879)

### Context

Docs-only PRs fail the Code Coverage CI job because tests are skipped
(no coverage artifacts generated), but the job unconditionally attempts
to download and publish those artifacts.

### Changes Made

Modified `.vsts-dotnet-ci.yml` CodeCoverage job to respect the
`onlyDocChanged` flag:

- Added dependency on `IfOnlyDocumentionChanged` job to access the flag
- Added `onlyDocChanged` variable to job variables
- Conditioned all artifact downloads on `eq(variables.onlyDocChanged,
0)`
- Conditioned coverage processing and publishing on
`eq(variables.onlyDocChanged, 0)`

This aligns with how other jobs (BootstrapMSBuildOnFullFrameworkWindows,
CoreBootstrappedOnLinux, etc.) already handle docs-only PRs.

### Testing

Pipeline configuration change - will be validated on next PR run. When
`onlyDocChanged = 1`, all coverage tasks skip; when `onlyDocChanged =
0`, coverage runs normally.

### Notes

Pre-existing typos in job/variable names (`Documention`, `Varibale`)
intentionally preserved for consistency across the pipeline.

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>docs only PRs fail code coverage CI check</issue_title>
> <issue_description>#12805
> #12857
> 
> ```
> Code Coverage failed
> [3 errors / 0
warnings](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1217301)
> 
> Annotations
> [Check failure on line 9 in Build
log](https://github.com/dotnet/msbuild/pull/12857/files#annotation_42398343450)
> 
> @azure-pipelines
> azure-pipelines
> / msbuild-pr (Code Coverage)
> Build log #L9
> 
> Publishing build artifacts failed with an error: Not found
PathtoPublish: D:\a\1\s\artifacts\CoverageResults\merged.coverage
> [Check failure on line 11 in Build
log](https://github.com/dotnet/msbuild/pull/12857/files#annotation_42398343487)
> 
> @azure-pipelines
> azure-pipelines
> / msbuild-pr (Code Coverage)
> Build log #L11
> 
> Artifact LinuxCoreCoverage was not found for build 1217301.
> [Check failure on line 9 in Build
log](https://github.com/dotnet/msbuild/pull/12857/files#annotation_42398343497)
> 
> @azure-pipelines
> azure-pipelines
> / msbuild-pr (Code Coverage)
> Build log #L9
> 
> Publishing build artifacts failed with an error: Not found
PathtoPublish: D:\a\1\s\artifacts\CoverageResults\merged.cobertura.xml
> ```
> 
> This is probably because testing is skipped on docs only PRs, in that
case code coverage should also be skipped</issue_description>
> 
> <agent_instructions>fix this issue, note that the CI pipeline
definition is called .vsts-dotnet-ci.yml</agent_instructions>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes #12878

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: JanProvaznik <[email protected]>

Author: Copilot

.vsts-dotnet-ci.yml

@@ -533,11 +533,14 @@ jobs:
 - job: CodeCoverage
   displayName: "Code Coverage"
   dependsOn:
+  - IfOnlyDocumentionChanged
   - BootstrapMSBuildOnFullFrameworkWindows
   - BootstrapMSBuildOnCoreWindows
   - FullReleaseOnWindows
   - CoreBootstrappedOnLinux
   - CoreOnMac
+  variables:
+    onlyDocChanged: $[ dependencies.IfOnlyDocumentionChanged.outputs['SetIfOnlyDocumentionChangedVaribale.onlyDocChanged'] ]
   pool:
     vmImage: 'windows-2022'
   steps:
@@ -562,32 +565,36 @@ jobs:
       buildType: 'current'
       artifactName: 'LinuxCoreCoverage'
       targetPath: '$(Build.SourcesDirectory)/artifacts/TestResults/CoverageResults/LinuxCore'
+    condition: eq(variables.onlyDocChanged, 0)
   - task: DownloadPipelineArtifact@2
     inputs:
       buildType: 'current'
       artifactName: 'MacCoreCoverage'
       targetPath: '$(Build.SourcesDirectory)/artifacts/TestResults/CoverageResults/MacCore'
+    condition: eq(variables.onlyDocChanged, 0)
 
   - task: PowerShell@2
     displayName: Process coverage reports
     inputs:
       filePath: $(Build.SourcesDirectory)\eng\process-coverage.ps1
       arguments: -repoRoot $(Build.SourcesDirectory) -coverageArtifactsDir $(Build.SourcesDirectory)/artifacts/CoverageResults
       pwsh: true
+    condition: eq(variables.onlyDocChanged, 0)
   - task: PublishBuildArtifacts@1
     displayName: Publish Artifact $(Build.BuildNumber) Coverage
     inputs:
       PathtoPublish: '$(Build.SourcesDirectory)/artifacts/CoverageResults/merged.coverage'
       ArtifactName: '$(Build.BuildNumber) Coverage'
-    condition: succeededOrFailed()
+    condition: and(succeededOrFailed(), eq(variables.onlyDocChanged, 0))
   - task: PublishBuildArtifacts@1
     displayName: Publish Artifact $(Build.BuildNumber) Cobertura
     inputs:
       PathtoPublish: '$(Build.SourcesDirectory)/artifacts/CoverageResults/merged.cobertura.xml'
       ArtifactName: '$(Build.BuildNumber) Cobertura'
-    condition: succeededOrFailed()
+    condition: and(succeededOrFailed(), eq(variables.onlyDocChanged, 0))
   - task: PublishCodeCoverageResults@2
     inputs:
       summaryFileLocation: '$(Build.SourcesDirectory)/artifacts/CoverageResults/merged.coverage'
       pathToSources: $(Build.SourcesDirectory)
+    condition: eq(variables.onlyDocChanged, 0)
 - template: /eng/common/templates/jobs/source-build.yml