chore: update dependabot configuration to monitor multiple NuGet directories and add GitHub Actions group

[jongalloway]

This pull request updates the .github/dependabot.yml configuration to improve dependency management for both the main and test projects. The changes expand monitoring coverage and introduce new grouping strategies for dependencies, making it easier to manage and review updates.

Dependabot configuration improvements:

• Added monitoring for NuGet dependencies in both /DotNetMcp and /DotNetMcp.Tests directories, instead of only /DotNetMcp.
• Introduced groups for NuGet dependencies, organizing updates by Microsoft dependencies, MCP SDK dependencies, and test dependencies (e.g., xunit).
• Added a group for GitHub Actions dependencies, grouping all action updates together.

[Copilot]

Pull request overview

Updates the repository’s Dependabot configuration to reduce dependency-update PR noise while expanding coverage to include both the main .NET project and its test project.

Changes:

• Switch NuGet updates from a single directory to directories to monitor both /DotNetMcp and /DotNetMcp.Tests.
• Add Dependabot grouping rules for NuGet dependencies (Microsoft., ModelContextProtocol, xunit.) to consolidate related updates.
• Add a Dependabot group for GitHub Actions updates to consolidate action version bumps.

Show a summary per file

File	Description
.github/dependabot.yml	Adds multi-directory NuGet monitoring and introduces grouping for NuGet and GitHub Actions updates.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0