Fix CI template test conflicts by implementing template isolation

[Copilot]

The CI job for local development validation was disabled due to conflicts with concurrent template tests. When both the Helix template tests and the local development validation job ran simultaneously, they would interfere with each other's template engine state, causing build failures.

Root Cause

The issue occurred because two different template testing workflows were accessing shared global state:

1. Helix template tests (in default-build.yml) already used proper isolation via --debug:custom-hive
2. Local development validation (in ci-public.yml) used global template installation in the user profile

This led to race conditions when both jobs tried to install/uninstall templates concurrently:

# Problematic: Global template installation
dotnet new install $PackagePath

Solution

Modified src/ProjectTemplates/scripts/Test-Template.psm1 to use the same isolation strategy as the Helix template tests:

# Isolated: Each test run gets its own template hive
$CustomHivePath = "$PSScriptRoot/.templatehive-$templateName"
dotnet new install $PackagePath --debug:disable-sdk-templates --debug:custom-hive $CustomHivePath
dotnet new $templateName --debug:custom-hive $CustomHivePath --debug:disable-sdk-templates

Key improvements:

• Unique custom hive per test run prevents template installation conflicts
• SDK template exclusion ensures complete isolation from system templates
• Automatic cleanup in finally blocks prevents state leakage between runs
• Process-scoped environment variables avoid global pollution

The solution mirrors the proven isolation pattern already used by Helix template tests, ensuring both workflows can run concurrently without conflicts.

Testing

• Verified PowerShell module syntax loads correctly
• Template isolation commands match existing successful patterns in the codebase
• Re-enabled the previously disabled CI job by changing condition: 'false' to condition: 'true'

Fixes #63149.

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