Onboard repository to Dependabot for automated testing and AI dependency updates

[Copilot]

Successfully onboarded the repository to Dependabot for automated dependency updates, refined based on maintainer feedback to target testing dependencies plus critical Microsoft.Extensions.AI packages with optimal organization.

What's Added

Dependabot Configuration (.github/dependabot.yml)

• Testing Dependency Monitoring: Automatically tracks testing dependencies from Directory.Packages.props that use version variables (e.g., $(System9Version))
• Microsoft.Extensions.AI Monitoring: Ensures Microsoft.Extensions.AI packages with version variables are updated in lockstep with the library
• GitHub Actions Monitoring: Keeps workflow actions updated for security and compatibility
• Weekly Schedule: Updates run every Monday at 06:00 UTC to balance freshness with maintenance overhead

Focused Package Grouping

To minimize PR noise, related packages are grouped together:

• Microsoft Extensions AI: Microsoft.Extensions.AI.Abstractions and Microsoft.Extensions.AI (using version variables)
• Testing Frameworks: xunit, Microsoft.NET.Test.Sdk, coverlet, Moq, GitHubActionsTestLogger
• Microsoft Extensions Testing: Microsoft.Extensions packages used in tests (DependencyInjection, Hosting, Logging, Logging.Console, Options - all using version variables)
• OpenTelemetry Testing: OpenTelemetry packages for testing and samples
• Serilog Testing: Serilog packages for testing and samples
• Other Testing: Individual testing packages like Anthropic.SDK, JsonSchema.Net, etc.

Key Features

• Central Package Management: Fully supports the repository's Directory.Packages.props with version variables like $(System9Version)
• Multi-Framework Compatible: Works seamlessly with netstandard2.0, net8.0, and net9.0 targets
• Controlled Updates: Limited to 5 concurrent dependency PRs and 5 GitHub Actions PRs
• Proper Labeling: All PRs tagged with dependencies and testing or github-actions
• Security Focused: Ensures timely updates for security vulnerabilities in testing infrastructure
• Clean Configuration: Uses group-based approach with wildcard ignore patterns for maintainability
• Version Variable Focus: Only monitors packages using version variables that benefit from automated coordination, excluding packages with fixed version strings that require manual updates

Scope

Included: Testing dependencies and Microsoft.Extensions.AI packages that use version variables:

• Test frameworks (xunit, Moq, coverlet)
• Microsoft.Extensions packages used in tests with $(System9Version)
• Microsoft.Extensions.AI.Abstractions and Microsoft.Extensions.AI with $(MicrosoftExtensionsAIVersion)
• OpenTelemetry packages for samples/testing
• Serilog packages for samples/testing
• Development tools (Anthropic.SDK, JsonSchema.Net, etc.)

Excluded:

• Product dependencies with fixed version strings (Microsoft.Extensions.Hosting.Abstractions, System.Text.Json, etc.)
• Packages requiring manual coordination (Microsoft.Extensions.TimeProvider.Testing at 9.5.0, Microsoft.Extensions.AI.OpenAI at 9.8.0-preview)
• Framework-specific packages for netstandard2.0, net8.0, net9.0
• Build infrastructure packages (Microsoft.SourceLink.GitHub)

Benefits

1. Automated Security Updates: Critical security patches applied automatically to testing infrastructure
2. AI Package Synchronization: Microsoft.Extensions.AI packages with version variables stay in sync with library development
3. Reduced Maintenance Overhead: Grouped updates minimize individual PR reviews
4. Compatibility Assurance: Regular updates help catch breaking changes early
5. Product Stability: Core product dependencies and packages with fixed versions remain stable through manual management
6. Coordination Flexibility: Packages requiring manual framework version updates (e.g., $(System9Version) to $(System10Version)) are excluded since they need coordinated changes anyway

The configuration ensures Microsoft.Extensions.AI packages are kept current while maintaining careful control over product dependencies and excluding packages with fixed versions that require manual coordination with framework updates.

Fixes #799.

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