This PR implements the ability for conventions to contribute custom content to PSRule job summaries, providing extensibility for the generated output.

Changes Made

Core Implementation

• Added IJobSummaryContributor interface: Allows conventions to provide custom content sections
• Added JobSummarySection class: Represents custom sections with title and markdown content
• Enhanced JobSummaryWriter: Now collects and displays convention contributions in job summaries
• Updated AssertPipelineBuilder: Passes convention contributors to JobSummaryWriter during pipeline construction

Key Features

• Backward compatible: Existing conventions and job summaries continue to work unchanged
• Extensible: Conventions can add multiple sections with custom titles and markdown content
• Error resilient: Exceptions from individual conventions don't break the entire job summary
• Flexible content: Supports any markdown content including tables, lists, links, code blocks, etc.

Usage Example

Conventions can implement the IJobSummaryContributor interface to add custom sections:

public class MyConvention : BaseConvention, IConventionV1, IJobSummaryContributor
{
    public IEnumerable<JobSummarySection>? GetJobSummaryContent()
    {
        return new[] 
        {
            new JobSummarySection("Custom Metrics", "- Processed: 100 items\n- Success rate: 95%"),
            new JobSummarySection("Environment Info", "- Runtime: .NET 8.0\n- OS: Linux")
        };
    }
    
    // ... other convention implementation
}

The job summary will include additional sections after the standard PSRule content:

# PSRule result summary

❌ PSRule completed with an overall result of 'Fail' with 10 rule(s) and 5 target(s) in 00:00:02.123.

## Analysis
...

## Custom Metrics
- Processed: 100 items
- Success rate: 95%

## Environment Info
- Runtime: .NET 8.0
- OS: Linux

Testing

• Added comprehensive unit tests covering normal operation, empty contributors, and error scenarios
• All existing tests continue to pass, ensuring backward compatibility
• Created example convention demonstrating the feature

This implementation provides the requested extensibility for job summaries while maintaining all existing functionality and following PSRule's architectural patterns.

Fixes #2667.

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