.NET: Update Aspire package to be preview

[westey-m]

Motivation and Context

This package should be released as preview, so updating settings

Description

• Update Aspire package to be preview

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

Updates the Aspire DevUI hosting package project to use the repo’s centralized NuGet packaging/versioning settings and marks it as a preview package.

Changes:

• Import dotnet/nuget/nuget-package.props for centralized package versioning/metadata defaults.
• Set VersionSuffix to preview and add NuGet metadata fields (Title/Tags/Description/Readme).
• Reorganize the solution entry for the DevUI project within agent-framework-dotnet.slnx.

Reviewed changes

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

File	Description
dotnet/src/Aspire.Hosting.AgentFramework.DevUI/Aspire.Hosting.AgentFramework.DevUI.csproj	Switches to centralized NuGet packaging props and sets preview/readme metadata.
dotnet/agent-framework-dotnet.slnx	Moves the DevUI project entry under the main /src/ folder grouping.

[github-actions]

Automated Code Review

Reviewers: 4 | Confidence: 95% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Design Approach

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Automated review by westey-m's agents