[ci] Make dnceng-public the default PR pipeline; disable PR triggers on DevDiv pipeline

[jonathanpeppers]

Context

Today PR validation runs on the DevDiv-internal Azure DevOps pipeline Xamarin.Android-PR (defined by build-tools/automation/azure-pipelines.yaml). The dnceng-public pipeline dotnet-android (defined by build-tools/automation/azure-pipelines-public.yaml) already runs the full PR test matrix, but every test stage has been gated on eq(variables['System.PullRequest.IsFork'], 'True') since #11153 ("Only run dnceng-public pipeline for fork PRs"), so it only exercises that matrix for fork PRs.

Build dnceng-public 1443410 (PR #11531, ~2h 18m, all green) demonstrates full matrix coverage on dnceng-public for a fork PR: Mac MSBuild 1-10, Mac MSBuild+Emulator 1-8, Mac APKs 1-2, Windows MSBuild 1-8, Linux MSBuild 1-2.

Changes

build-tools/automation/azure-pipelines-public.yaml:

• Drop condition: eq(variables['System.PullRequest.IsFork'], 'True') from the mac_build, win_build_test, and linux_build stages.
• Drop the IsFork clause from the conditions on the three test stages (linux_tests, mac_msbuild_tests, mac_emulator_tests); they keep succeeded() and the SkipTestStages guard.
• Change the skipTests parameter default from 'auto' to 'false'.
• Collapse the SkipTestStages variable expression to just the explicit true/else: false cases (the IsFork-aware auto branch is gone).

build-tools/automation/azure-pipelines.yaml:

• Add pr: none after the existing trigger: block so the DevDiv pipeline stops triggering on PRs. The trigger: list (main, d16-*, d17-*, release/*, feature/*) is unchanged, so CI on main/release branches and official signed builds continue to run normally.

Verification

• git grep -n IsFork build-tools/automation/azure-pipelines-public.yaml → zero hits.
• git grep -n "pr: none" build-tools/automation/azure-pipelines.yaml → one hit.
• Both YAML files parse cleanly with yaml.safe_load.
• azure-pipelines.yaml trigger: block is untouched.

Follow-up (manual repo-admin step)

After this merges, branch policies in the dotnet/android repo settings must be updated to require the dotnet-android (dnceng-public) check instead of Xamarin.Android-PR. That has to be done by a maintainer in the repo admin UI alongside merging this PR.

[Copilot]

Pull request overview

Updates the Azure DevOps pipeline configuration to make the dnceng-public dotnet-android pipeline the default PR validation path, and to stop the DevDiv pipeline from triggering on PRs (while keeping branch CI triggers intact).

Changes:

• Disabled PR triggers on the DevDiv pipeline by adding pr: none to build-tools/automation/azure-pipelines.yaml.
• Removed fork-only stage gating in azure-pipelines-public.yaml so the full PR matrix runs for non-fork PRs as well.
• Changed the public pipeline skipTests default to 'false' and simplified SkipTestStages to explicit true/false behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
build-tools/automation/azure-pipelines.yaml	Disables PR triggering for the DevDiv pipeline via pr: none, keeping existing branch triggers.
build-tools/automation/azure-pipelines-public.yaml	Enables full PR matrix by removing fork-only stage conditions and simplifying test-skip parameter handling.

Comments suppressed due to low confidence (1)

build-tools/automation/azure-pipelines-public.yaml:26

• 💡 Suggestion: skipTests still allows an 'auto' value, but the SkipTestStages logic no longer has any special handling for it (it now behaves the same as 'false'). To avoid confusing/unsupported UI options, consider removing 'auto' from the allowed values (or reintroducing explicit auto semantics).

  default: 'false'
  values:
  - 'auto'
  - 'true'
  - 'false'