Make IIssue immutable by removing mutable Run and FileLink properties

[Copilot]

This PR restores immutability to the IIssue interface by making the Run and FileLink properties read-only and implementing a copy-based enhancement pattern in IssuesReader.

Problem

The IIssue interface was no longer immutable due to two properties that were settable:

• Run { get; set; }
• FileLink { get; set; }

These properties couldn't be made read-only in the original design because they are not set by issue providers during construction, but rather generically for all issue providers by the IssuesReader after creation.

Solution

Following the approach suggested in the issue, this PR implements cloning and enhancement of issues in the IssuesReader:

1. Made properties immutable: Changed Run and FileLink to { get; } in both IIssue interface and Issue class
2. Added copy method: Implemented Issue.WithRunAndFileLink() that creates a new Issue instance with updated Run and FileLink values
3. Updated IssuesReader: Modified to create enhanced copies instead of mutating original issues
4. Fixed tests: Updated all affected tests to compare issues by identifier instead of reference equality

Key Changes

Core immutability changes:

// Before (mutable)
public interface IIssue 
{
    string Run { get; set; }
    Uri FileLink { get; set; }
}

// After (immutable)
public interface IIssue 
{
    string Run { get; }
    Uri FileLink { get; }
}

Copy-based enhancement:

public Issue WithRunAndFileLink(string run, Uri fileLink) =>
    new(/* all existing properties + new run and fileLink */);

IssuesReader enhancement pattern:

// Creates new enhanced issues instead of mutating originals
foreach (var issue in currentIssues)
{
    var enhancedIssue = originalIssue.WithRunAndFileLink(run, fileLink);
    enhancedIssues.Add(enhancedIssue);
}

Benefits

• ✅ Immutability restored: IIssue objects cannot be modified after creation
• ✅ Thread safety: Immutable objects are inherently thread-safe
• ✅ Type safety: Compile-time enforcement prevents accidental mutations
• ✅ Backward compatibility: Issue providers require no changes
• ✅ Functionality preserved: All existing features work unchanged

Testing

All 3000+ tests pass, including comprehensive test updates to handle the new immutable behavior. Tests now compare issues by content (identifier) rather than reference equality since the reader returns enhanced copies.

Fixes #194.

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