[CI] Add nightly ARM64 tests for Process Containers and Isolation Session

[Elliot (theelliotm)]

📖 Description

This PR adds jobs that run Process Container and Isolation Session tests in ARM64 images during the nightly CI Scheduled Validation workflow. They mirror their corresponding Windows x64 jobs, with the exception of 23H2 which does not have an ARM image available for testing currently.

Other Changes

• Powershell 7, Node, and Python 3 now install during each job via winget. This helps for compat with FlowEval.
• Removed Hyperlight mentions
• Documentation brought up-to-date

🔗 References

🔍 Validation

Tested in CI: https://github.com/microsoft/mxc/actions/runs/35917329987/job/107378599796
Process Container and Isolation Session tests function identically.

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)
• If this PR changes Cargo.lock, the dependency-feed-check check passes (see docs/pull-requests.md)

📋 Issue Type

• Bug fix
• Feature
• Task

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Copilot review overview

🟡 Changes recommended

Incorrect WinGet installer filters can prevent required tooling installation and fail CI jobs.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 1 · 2

Open (3)

• Use the MSI installer type for PowerShell · New
• Use the correct 24H2 ARM64 runner pool · New
• Remove hyperlight from Linux backend usage text · New

What changed in this PR

Adds nightly Windows ARM64 validation for Process Container and Isolation Session workloads.

Changes:

• Enables eligible ARM64 Windows matrix jobs.
• Installs PowerShell, Node, and Python through WinGet.
• Removes Hyperlight validation support and updates documentation.

File	Review
scripts/​ci/​validation-test-matrix.json	Adds ARM64 Windows pools and capabilities.
scripts/​ci/​run_backend_validation_tests.sh	Removes Hyperlight dispatch, but usage text still advertises it (nit).
scripts/​ci/​run_backend_validation_tests.ps1	Removes Hyperlight dispatch, but referenced documentation remains stale (nit).
scripts/​ci/​resolve-validation-test-matrix.mjs	Enables eligible ARM64 matrix expansion.
scripts/​ci/​prepare-windows-host.ps1	Adds tooling installation; incorrect WinGet installer filters block PowerShell (critical), Node, and Python (moderate).
scripts/​ci/​prepare-linux-host.sh	Removes Hyperlight preparation, but referenced documentation remains stale (nit).
docs/​ci-validation-infrastructure.md	Contains inaccurate timeouts, capabilities, ARM64 pools/availability, and WSLC behavior (nits).
.github/​workflows/​Validation.Tests.Matrix.Job.yml	Changes Windows preparation behavior and timeout; documentation remains inconsistent (nit).

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[Copilot]

Copilot review overview

🔵 Needs a closer look

CI documentation remains inconsistent with the matrix, timeout, and removed Hyperlight dispatcher support.

Review effort: Balanced
Findings: None

Resolved since last review (3)

• Use the MSI installer type for PowerShell
• Remove hyperlight from Linux backend usage text
• Use the correct 24H2 ARM64 runner pool

Previously missed (1)

In code that hasn't changed since last review

Document accurate x64 capabilities for 25H2 and 24H2

docs/​ci-validation-infrastructure.md:112

The documented capabilities for 25H2 and 24H2 do not match the catalog: their x64 backends arrays contain process-t1, wslc, windows-sandbox, and microvm, with no isolation-session; only the ARM64 arrays add isolation-session. Replacing same as above with the actual lists is necessary so this table does not advertise an unsupported x64 capability.

[Copilot]

Copilot review overview

🔵 Needs a closer look

CI documentation remains inconsistent with the backend catalog and dispatcher behavior.

Review effort: Balanced
Findings: 1

Open (1)

• Correct architecture-specific backend declarations · New

Previously missed (1)

In code that hasn't changed since last review

Update CI docs after removing hyperlight backend

scripts/​ci/​run_backend_validation_tests.ps1:18

This removes hyperlight from the dispatcher, but tests/scripts/README.md:98-99 still documents hyperlight as an accepted CI backend for these two dispatchers. That leaves the CI documentation contradictory: following the documented backend id now fails validation before any suite runs. Update the dispatcher documentation in the same change.

[Copilot]

Copilot review overview

🔵 Needs a closer look

The capability documentation contradicts the validation catalog and related dispatch documentation remains stale.

Review effort: Balanced
Findings: 1

Open (1)

• Correct architecture-specific backend declarations

[Copilot]

Copilot review overview

🟡 Changes recommended

The Python installation failure and inaccurate ARM64 documentation must be corrected before approval.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 1

Open (1)

• Use the Python package's supported installer type · New

Resolved since last review (1)

• Correct architecture-specific backend declarations

Previously missed (1)

In code that hasn't changed since last review

Remove hyperlight from dispatcher documentation

scripts/​ci/​run_backend_validation_tests.ps1:18

After removing hyperlight from both validation dispatchers, tests/scripts/README.md still advertises it as a supported backend for each dispatcher. That documentation now causes users to select an ID rejected by these entry points; please update the dispatcher table in the same change.

[Copilot]

Copilot review overview

🔵 Needs a closer look

The ARM64 capability documentation must list only the supported backends.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Use the Python package's supported installer type