.github: Add Mu PR Validation workflow

[makubacki]

Description

Adds workflows to run QEMU-based platform validation on pull requests to mu_plus. 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

This workflow differs slightly from the Mu Basecore equivalent file in that it is triggered by the CodeQL workflow instead of the CLANGPDB Package CI workflow used there. The reason is that the CLANGPDB Package CI workflow was not added to mu_plus since mu_plus requires Rust code to be built and that workflow doesn't include Rust build support. The CodeQL workflow in mu_plus serves a similar purpose by verifying local package compilation prior to attempting a mu_tiano_platforms build with the changes.

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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_plus fork.

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):

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@1931cff). Learn more about missing BASE report.

Additional details and impacted files

@@                Coverage Diff                @@
##             release/202511     #865   +/-   ##
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  Coverage                  ?   10.10%           
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  Files                     ?      149           
  Lines                     ?    24833           
  Branches                  ?     2346           
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  Hits                      ?     2509           
  Misses                    ?    22288           
  Partials                  ?       36           

Flag	Coverage Δ
AdvLoggerPkg	2.86% <ø> (?)
HidPkg	2.79% <ø> (?)
MfciPkg	34.00% <ø> (?)
MsCorePkg	1.30% <ø> (?)
MsWheaPkg	7.37% <ø> (?)
XmlSupportPkg	25.46% <ø> (?)

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[makubacki]

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