[release/10.0] Update Ubuntu container references from 25.10 to 26.04

[Copilot]

Fixes Issue

main PR #121730

Description

Backports Ubuntu container version updates from main. Changes Ubuntu 25.10 (preview during initial addition) to 26.04 (now in preview with official container releases) for Helix test infrastructure.

Updates three container references:

• eng/pipelines/installer/helix-queues-setup.yml: Linux ARM64
• eng/pipelines/libraries/helix-queues-setup.yml: Linux ARM64 and AMD64

All changes target container images only (@mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-26.04-*). VM references remain unchanged. The helix-platforms.yml file does not exist in release/10.0.

Customer Impact

No direct customer impact. Infrastructure change to align test environments with current Ubuntu preview release.

Regression

No. This is a proactive infrastructure update.

Testing

Changes are configuration-only. CI validation in main has been completed per the original PR.

Risk

Low. Container references already validated in main branch. No code changes, only test infrastructure configuration.

Original prompt

Please apply PR #121730 to the release/10.0 branch. I don't think the helix-platforms.yml file exists there. Please do not create it.

#121730

The goal after this change is that all Ubuntu container references are using Ubuntu 26.04. Do not change VM references. Those will be handled separately.

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[richlander]

This change is good to go. All failures are on Android change is Linux-specific.

Note that this change switched Ubuntu testing to 26.04, which is still in preview state. I'm unaware that this has been a problem in main and don't expect it.

I'll do the same with release/8.0-staging and release/9.0-staging but will wait until 2026 for that.

[richlander]

/ba-g "Failures are unrelated, on Android only"

[Copilot]

Pull request overview

This PR backports Ubuntu container version updates from main (PR #121730), updating container image references from Ubuntu 25.10 (which was in preview during initial addition) to Ubuntu 26.04 (currently in preview with official container releases). This is a proactive infrastructure update to align Helix test environments with the current Ubuntu preview release.

• Updates three container references from Ubuntu 25.10 to 26.04
• Changes affect only container images (@mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-26.04-*)
• VM references remain unchanged as intended

Reviewed changes

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

File	Description
eng/pipelines/libraries/helix-queues-setup.yml	Updates Ubuntu container references for both ARM64 (line 34) and AMD64 (line 67) architectures in library test infrastructure
eng/pipelines/installer/helix-queues-setup.yml	Updates Ubuntu container reference for ARM64 architecture (line 32) in installer test infrastructure

[richlander]

Using tell mode for this.