Refactor foreach loop to use LINQ Select in PostTestDataBuilder

[Copilot]

Addresses code review feedback from PR #116 to use .Select() instead of foreach with immediate variable mapping.

Changes

• Refactored WithPostReplyingTo() in PostTestDataBuilder to use LINQ projection

// Before
foreach (var mentionedPostId in mentionedPostIds)
{
    var postToReply = new PostToReply { PostId = mentionedPostId, Reply = post };
    _dbContext.PostsToReplies.Add(postToReply);
}

// After
var postsToReplies = mentionedPostIds.Select(mentionedPostId => new PostToReply
{
    PostId = mentionedPostId,
    Reply = post,
});
_dbContext.PostsToReplies.AddRange(postsToReplies);

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[Copilot]

Pull request overview

This PR refactors the WithPostReplyingTo() method in PostTestDataBuilder to replace a foreach loop with LINQ's Select() method, making the code more functional and concise.

Key Changes:

• Replaced foreach loop with Select() LINQ projection for creating PostToReply objects
• Changed from multiple Add() calls to a single AddRange() call

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