Store polyglot artifacts under the workspace .aspire folder

[IEvangelist]

Summary

• resolve the polyglot workspace root from aspire.config.json (or legacy .aspire/settings.json) and store workspace-scoped CLI artifacts under <workspace>/.aspire/..., including bundled restore output and prebuilt AppHost staging
• flow an explicit temporary nuget.config into bundle restore for explicit channels and make the restore cache source-aware so same-version packages from different feeds no longer collide under the workspace .aspire restore cache
• add a project-root .gitignore entry for .aspire/ to generated TypeScript, Python, and Java projects so the new workspace-local artifact folder stays untracked by default
• preserve an existing root .gitignore when scaffolding a polyglot AppHost by appending missing ignore entries instead of overwriting the file, and fail scaffolding if any other returned project files already exist

Fixes #15967.
Addresses #15399.

Testing

• dotnet test tests/Aspire.Cli.Tests/Aspire.Cli.Tests.csproj -- --filter-class Aspire.Cli.Tests.NuGet.BundleNuGetServiceTests --filter-class Aspire.Cli.Tests.Packaging.TemporaryNuGetConfigTests --filter-class Aspire.Cli.Tests.Projects.PrebuiltAppHostServerTests
• dotnet test tests/Aspire.Cli.Tests/Aspire.Cli.Tests.csproj -- --filter-class Aspire.Cli.Tests.Templating.TemplateGitIgnoreTests --filter-class Aspire.Cli.Tests.Scaffolding.ScaffoldingServiceTests
• dotnet build tests/Aspire.Cli.EndToEnd.Tests/Aspire.Cli.EndToEnd.Tests.csproj
• manual CLI smoke: �spire new ts-starter and �spire new ts generated a project-root .gitignore containing the .aspire/ entry

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 15991

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 15991"

[github-actions]

Re-running the failed jobs in the CI workflow for this pull request because 1 job was identified as retry-safe transient failures in the CI run attempt.
GitHub was asked to rerun all failed jobs for that attempt, and the rerun is being tracked in the rerun attempt.
The job links below point to the failed attempt jobs that matched the retry-safe transient failure rules.

• Tests / Hosting-1 / Hosting-1 (windows-latest) - Job-level runner or infrastructure failure matched the transient allowlist.

[Copilot]

Copilot reviewed 23 out of 23 changed files in this pull request and generated no comments.

[github-actions]

Re-running the failed jobs in the CI workflow for this pull request because 2 jobs were identified as retry-safe transient failures in the CI run attempt.
GitHub was asked to rerun all failed jobs for that attempt, and the rerun is being tracked in the rerun attempt.
The job links below point to the failed attempt jobs that matched the retry-safe transient failure rules.

• Tests / Microsoft.EntityFrameworkCore.Cosmos / Microsoft.EntityFrameworkCore.Cosmos (windows-latest) - Job-level runner or infrastructure failure matched the transient allowlist.
• Tests / Hosting-5 / Hosting-5 (windows-latest) - Job-level runner or infrastructure failure matched the transient allowlist.