feat: Add support for message attachments

- Introduced Attachment model to handle file attachments associated with messages.
- Updated ModerationController to manage message deletions and attachment cleanup.
- Enhanced ChatService to include attachments in message retrieval.
- Implemented migration for legacy single-attachment messages to the new Attachments model.
- Added unit tests for attachment handling in message formatting and parsing.
- Updated database context and migrations to support new Attachments table.

src/EchoHub.Tests/DroppedFileParserTests.cs

@@ -0,0 +1,116 @@
+using EchoHub.Client.UI.Helpers;
+using Xunit;
+
+namespace EchoHub.Tests;
+
+public class DroppedFileParserTests
+{
+    // ── LooksLikePath ─────────────────────────────────────────────────
+
+    [Theory]
+    [InlineData("C:\\Users\\me\\cat.png")]
+    [InlineData("D:/photos/pic.jpg")]
+    [InlineData("\"C:\\My Files\\a b.png\"")]
+    [InlineData("/home/me/song.mp3")]
+    [InlineData("\\\\server\\share\\file.txt")]
+    public void LooksLikePath_PathLikeInput_ReturnsTrue(string text)
+    {
+        Assert.True(DroppedFileParser.LooksLikePath(text));
+    }
+
+    [Theory]
+    [InlineData("hello world")]
+    [InlineData("check out my cat")]
+    [InlineData("no")]
+    [InlineData("")]
+    [InlineData("@someone hi")]
+    public void LooksLikePath_NormalChat_ReturnsFalse(string text)
+    {
+        Assert.False(DroppedFileParser.LooksLikePath(text));
+    }
+
+    // ── TryGetFiles (injected existence check) ────────────────────────
+
+    [Fact]
+    public void TryGetFiles_SingleWindowsPath_Detected()
+    {
+        var exists = Exists("C:\\Users\\me\\cat.png");
+        Assert.True(DroppedFileParser.TryGetFiles("C:\\Users\\me\\cat.png", out var files, exists));
+        Assert.Equal(["C:\\Users\\me\\cat.png"], files);
+    }
+
+    [Fact]
+    public void TryGetFiles_QuotedPathWithSpaces_StripsQuotes()
+    {
+        var path = "C:\\My Files\\a b.png";
+        Assert.True(DroppedFileParser.TryGetFiles($"\"{path}\"", out var files, Exists(path)));
+        Assert.Equal([path], files);
+    }
+
+    [Fact]
+    public void TryGetFiles_MultipleQuotedPaths_Detected()
+    {
+        var a = "C:\\a.png";
+        var b = "C:\\b.mp3";
+        Assert.True(DroppedFileParser.TryGetFiles($"\"{a}\" \"{b}\"", out var files, Exists(a, b)));
+        Assert.Equal([a, b], files);
+    }
+
+    [Fact]
+    public void TryGetFiles_PosixAbsolutePath_Detected()
+    {
+        // Path.IsPathFullyQualified treats "/x" as fully qualified only on non-Windows;
+        // this asserts the parser defers that judgment to the platform.
+        var isPosix = !OperatingSystem.IsWindows();
+        var detected = DroppedFileParser.TryGetFiles("/home/me/song.mp3", out var files, Exists("/home/me/song.mp3"));
+        Assert.Equal(isPosix, detected);
+        if (isPosix)
+            Assert.Equal(["/home/me/song.mp3"], files);
+    }
+
+    [Fact]
+    public void TryGetFiles_NonExistentPath_ReturnsFalse()
+    {
+        Assert.False(DroppedFileParser.TryGetFiles("C:\\nope\\missing.png", out _, _ => false));
+    }
+
+    [Fact]
+    public void TryGetFiles_PartialPathDuringTyping_ReturnsFalseUntilComplete()
+    {
+        // Only the fully typed path exists; prefixes do not.
+        var full = "C:\\Users\\me\\cat.png";
+        var exists = Exists(full);
+        Assert.False(DroppedFileParser.TryGetFiles("C:\\Users\\me\\ca", out _, exists));
+        Assert.True(DroppedFileParser.TryGetFiles(full, out _, exists));
+    }
+
+    [Fact]
+    public void TryGetFiles_OneMissingAmongMultiple_ReturnsFalse()
+    {
+        var a = "C:\\a.png";
+        Assert.False(DroppedFileParser.TryGetFiles($"\"{a}\" \"C:\\gone.png\"", out _, Exists(a)));
+    }
+
+    [Fact]
+    public void TryGetFiles_RealTempFile_DetectedWithDefaultExists()
+    {
+        var temp = Path.Combine(Path.GetTempPath(), $"echohub_drop_{Guid.NewGuid():N}.txt");
+        File.WriteAllText(temp, "x");
+        try
+        {
+            Assert.True(DroppedFileParser.TryGetFiles(temp, out var files));
+            Assert.Single(files);
+            Assert.Equal(temp, files[0]);
+        }
+        finally
+        {
+            File.Delete(temp);
+        }
+    }
+
+    private static Func<string, bool> Exists(params string[] existing)
+    {
+        var set = new HashSet<string>(existing, StringComparer.OrdinalIgnoreCase);
+        return set.Contains;
+    }
+}