.github: Add Mu PR Validation workflow

[makubacki]

Description

Adds workflows to run QEMU-based platform validation on pull requests to mu_basecore. This includes:

• mu-pr-validation.yml: The main workflow that performs the actual QEMU validation, including building and booting.

• mu-pr-validation-pending.yml: A workflow that posts an immediate "pending" notification on a pull request when it's pushed, indicating that QEMU validation is waiting for CI to complete. This is broken out to its own workflow to use the pull_request_target trigger (access to secrets on PRs from public forks) while minimizing the amount of code that runs with those elevated permissions.

• mu-pr-validation-post.yml: A workflow that posts the results of the QEMU validation run to the pull request once the validation is complete.

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Notes

1. Depends on Repo File Sync: Add EDKII CLANGPDB CI Workflow #1734 to be completed first since the CLANGPDB Package CI workflow is a trigger upon its completion.
2. The workflows here will call a reusable workflow on the add_mu_pr_val_workflow_e2e_val branch on mu_devops during this initial rollout period. This allows any necessary adjustments to be made quickly based on observations and feedback.

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• Impacts functionality?
• Impacts security?
• Breaking change?
• Includes tests?
• Includes documentation?

How This Was Tested

Ran the workflow under various conditions on my mu_basecore fork. Some examples of output are shown below.

Note that the "boot times" are not precise. They are the total time for the --flashonly step to run including stuart overhead. However, they are not too far off from the boot time and provide the ability to get a relative sense of perf impact.

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PENDING STATE (when the workflow is waiting for CI to finish)

IN PROGRESS (building / booting QEMU FW with the change)

Granular status updates (see ending text):

BUILD FAILURE

Status check result:

BOOT FAILURE

SUCCESS

Integration Instructions

• N/A - Only impacts PR validation in this repo

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release/202511@eca2e6e). Learn more about missing BASE report.

Additional details and impacted files

@@                Coverage Diff                @@
##             release/202511    #1737   +/-   ##
=================================================
  Coverage                  ?    2.21%           
=================================================
  Files                     ?     1632           
  Lines                     ?   419626           
  Branches                  ?     4949           
=================================================
  Hits                      ?     9308           
  Misses                    ?   410243           
  Partials                  ?       75           

Flag	Coverage Δ
FmpDevicePkg	9.53% <ø> (?)
MdeModulePkg	1.58% <ø> (?)
MdePkg	5.44% <ø> (?)
NetworkPkg	0.55% <ø> (?)
PolicyServicePkg	30.42% <ø> (?)
SecurityPkg	1.61% <ø> (?)
UefiCpuPkg	4.78% <ø> (?)
UnitTestFrameworkPkg	11.70% <ø> (?)

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[makubacki]

Note: Waiting until all clangpdb-ci.yml changes are in to merge: microsoft/mu_devops#553