feat: Add initial implementation of CI/CD workflows, documentation, and testing framework

src/EchoHub.Tests/EchoHub.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.*" />
+    <PackageReference Include="xunit" Version="2.*" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.*">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\EchoHub.Core\EchoHub.Core.csproj" />
+    <ProjectReference Include="..\EchoHub.Server\EchoHub.Server.csproj" />
+  </ItemGroup>
+
+</Project>

src/EchoHub.Tests/FileValidationHelperTests.cs

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+using EchoHub.Server.Services;
+using Xunit;
+
+namespace EchoHub.Tests;
+
+public class FileValidationHelperTests
+{
+    [Fact]
+    public void IsValidImage_JpegMagicBytes_ReturnsTrue()
+    {
+        byte[] jpeg = [0xFF, 0xD8, 0xFF, 0xE0, 0x00, 0x10];
+        using var stream = new MemoryStream(jpeg);
+        Assert.True(FileValidationHelper.IsValidImage(stream));
+    }
+
+    [Fact]
+    public void IsValidImage_PngMagicBytes_ReturnsTrue()
+    {
+        byte[] png = [0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A];
+        using var stream = new MemoryStream(png);
+        Assert.True(FileValidationHelper.IsValidImage(stream));
+    }
+
+    [Fact]
+    public void IsValidImage_GifMagicBytes_ReturnsTrue()
+    {
+        byte[] gif = [0x47, 0x49, 0x46, 0x38, 0x39, 0x61];
+        using var stream = new MemoryStream(gif);
+        Assert.True(FileValidationHelper.IsValidImage(stream));
+    }
+
+    [Fact]
+    public void IsValidImage_WebpMagicBytes_ReturnsTrue()
+    {
+        byte[] webp = [0x52, 0x49, 0x46, 0x46, 0x00, 0x00, 0x00, 0x00, 0x57, 0x45, 0x42, 0x50];
+        using var stream = new MemoryStream(webp);
+        Assert.True(FileValidationHelper.IsValidImage(stream));
+    }
+
+    [Fact]
+    public void IsValidImage_RandomBytes_ReturnsFalse()
+    {
+        byte[] random = [0x00, 0x01, 0x02, 0x03, 0x04, 0x05];
+        using var stream = new MemoryStream(random);
+        Assert.False(FileValidationHelper.IsValidImage(stream));
+    }
+
+    [Fact]
+    public void IsValidImage_ResetsStreamPosition()
+    {
+        byte[] jpeg = [0xFF, 0xD8, 0xFF, 0xE0, 0x00, 0x10];
+        using var stream = new MemoryStream(jpeg);
+        FileValidationHelper.IsValidImage(stream);
+        Assert.Equal(0, stream.Position);
+    }
+}

src/EchoHub.Tests/PresenceTrackerTests.cs

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+using EchoHub.Server.Services;
+using Xunit;
+
+namespace EchoHub.Tests;
+
+public class PresenceTrackerTests
+{
+    [Fact]
+    public void UserConnected_IsOnline_ReturnsTrue()
+    {
+        var tracker = new PresenceTracker();
+        tracker.UserConnected("conn1", Guid.NewGuid(), "alice");
+        Assert.True(tracker.IsOnline("alice"));
+    }
+
+    [Fact]
+    public void UserDisconnected_LastConnection_IsOnlineReturnsFalse()
+    {
+        var tracker = new PresenceTracker();
+        tracker.UserConnected("conn1", Guid.NewGuid(), "alice");
+        tracker.UserDisconnected("conn1");
+        Assert.False(tracker.IsOnline("alice"));
+    }
+
+    [Fact]
+    public void MultipleConnections_DisconnectOne_StillOnline()
+    {
+        var tracker = new PresenceTracker();
+        var userId = Guid.NewGuid();
+        tracker.UserConnected("conn1", userId, "alice");
+        tracker.UserConnected("conn2", userId, "alice");
+        tracker.UserDisconnected("conn1");
+        Assert.True(tracker.IsOnline("alice"));
+    }
+
+    [Fact]
+    public void JoinChannel_GetOnlineUsersInChannel_ReturnsUser()
+    {
+        var tracker = new PresenceTracker();
+        tracker.UserConnected("conn1", Guid.NewGuid(), "alice");
+        tracker.JoinChannel("alice", "general");
+        var users = tracker.GetOnlineUsersInChannel("general");
+        Assert.Contains("alice", users);
+    }
+
+    [Fact]
+    public void LeaveChannel_UserNoLongerInChannel()
+    {
+        var tracker = new PresenceTracker();
+        tracker.UserConnected("conn1", Guid.NewGuid(), "alice");
+        tracker.JoinChannel("alice", "general");
+        tracker.LeaveChannel("alice", "general");
+        var users = tracker.GetOnlineUsersInChannel("general");
+        Assert.DoesNotContain("alice", users);
+    }
+
+    [Fact]
+    public void GetChannelsForUser_ReturnsJoinedChannels()
+    {
+        var tracker = new PresenceTracker();
+        tracker.UserConnected("conn1", Guid.NewGuid(), "alice");
+        tracker.JoinChannel("alice", "general");
+        tracker.JoinChannel("alice", "random");
+        var channels = tracker.GetChannelsForUser("alice");
+        Assert.Contains("general", channels);
+        Assert.Contains("random", channels);
+    }
+}

src/EchoHub.Tests/ValidationConstantsTests.cs

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+using EchoHub.Core.Constants;
+using Xunit;
+
+namespace EchoHub.Tests;
+
+public class ValidationConstantsTests
+{
+    [Theory]
+    [InlineData("alice", true)]
+    [InlineData("Bob_123", true)]
+    [InlineData("user-name", true)]
+    [InlineData("abc", true)]
+    [InlineData("ab", false)]
+    [InlineData("", false)]
+    [InlineData("has space", false)]
+    [InlineData("has@symbol", false)]
+    public void UsernameRegex_ValidatesCorrectly(string input, bool expected)
+    {
+        var result = ValidationConstants.UsernameRegex().IsMatch(input);
+        Assert.Equal(expected, result);
+    }
+
+    [Theory]
+    [InlineData("general", true)]
+    [InlineData("my-channel_01", true)]
+    [InlineData("ab", true)]
+    [InlineData("a", false)]
+    [InlineData("has space", false)]
+    public void ChannelNameRegex_ValidatesCorrectly(string input, bool expected)
+    {
+        var result = ValidationConstants.ChannelNameRegex().IsMatch(input);
+        Assert.Equal(expected, result);
+    }
+
+    [Theory]
+    [InlineData("#FF0000", true)]
+    [InlineData("#aabbcc", true)]
+    [InlineData("FF0000", false)]
+    [InlineData("#FFF", false)]
+    [InlineData("#GGGGGG", false)]
+    public void HexColorRegex_ValidatesCorrectly(string input, bool expected)
+    {
+        var result = ValidationConstants.HexColorRegex().IsMatch(input);
+        Assert.Equal(expected, result);
+    }
+}