Backport macOS 26 Tahoe test fixes to release/9.0-staging

[vcsjones]

Backport #118652
Backport #118777

Customer Impact

None, test only change.

Regression

• Yes
• No

Testing

These tests were failing against macOS 26 RC. They now pass against macOS 26 RC.

Risk

None, test only change.

[Copilot]

Pull Request Overview

This PR backports test fixes for macOS 26 Tahoe compatibility to the release/9.0-staging branch. The changes address test failures that occurred when running against macOS 26 RC by updating platform detection logic and adjusting test expectations for certificate chain validation behavior on newer Apple platforms.

Key changes:

• Added platform detection for Apple platforms version 26 or later
• Updated certificate chain test expectations to handle behavioral differences in macOS 26
• Modified test logic to group Linux and Apple 26+ platforms with similar behavior patterns

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/Common/tests/TestUtilities/System/PlatformDetection.Unix.cs	Adds new property to detect Apple platforms version 26 or later
src/libraries/System.Security.Cryptography/tests/X509Certificates/ChainTests.cs	Updates certificate chain test expectations for macOS 26 compatibility

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-security, @bartonjs, @vcsjones
See info in area-owners.md if you want to be subscribed.

[bartonjs]

Test-only change, applying servicing-approved under tell-mode.

[vcsjones]

I've given that timing out pipeline 4 tries, not really sure a 5th is going to work.

[jeffhandley]

/ba-g Unrelated timeout