Expand coverage for transport, chat, and media flows (#1056)

* Expand coverage for transport, chat, and media flows

* Stabilize transport and media coverage tests

---------

Co-authored-by: jack <jackjackbits@users.noreply.github.com>

bitchatTests/Features/ImageUtilsTests.swift

@@ -0,0 +1,67 @@
+import Testing
+import Foundation
+#if os(iOS)
+import UIKit
+#else
+import AppKit
+#endif
+@testable import bitchat
+
+private func makeTemporaryFileURL(_ name: String) -> URL {
+    FileManager.default.temporaryDirectory.appendingPathComponent(name)
+}
+
+#if os(iOS)
+private func makePlatformImage(size: CGSize) -> UIImage {
+    UIGraphicsImageRenderer(size: size).image { context in
+        UIColor.systemTeal.setFill()
+        context.fill(CGRect(origin: .zero, size: size))
+    }
+}
+#else
+private func makePlatformImage(size: CGSize) -> NSImage {
+    let image = NSImage(size: size)
+    image.lockFocus()
+    NSColor.systemTeal.setFill()
+    NSBezierPath(rect: CGRect(origin: .zero, size: size)).fill()
+    image.unlockFocus()
+    return image
+}
+#endif
+
+struct ImageUtilsTests {
+    @Test
+    func processImage_rejectsOversizedSourceFile() throws {
+        let url = makeTemporaryFileURL("image-too-large.bin")
+        try Data(repeating: 0xFF, count: 10 * 1024 * 1024 + 1).write(to: url, options: .atomic)
+        defer { try? FileManager.default.removeItem(at: url) }
+
+        #expect(throws: ImageUtilsError.self) {
+            try ImageUtils.processImage(at: url)
+        }
+    }
+
+    @Test
+    func processImage_rejectsInvalidImageData() throws {
+        let url = makeTemporaryFileURL("image-invalid.bin")
+        try Data("not-an-image".utf8).write(to: url, options: .atomic)
+        defer { try? FileManager.default.removeItem(at: url) }
+
+        #expect(throws: ImageUtilsError.self) {
+            try ImageUtils.processImage(at: url)
+        }
+    }
+
+    @Test
+    func processImage_writesCompressedJpeg() throws {
+        let image = makePlatformImage(size: CGSize(width: 1024, height: 768))
+        let outputURL = try ImageUtils.processImage(image, maxDimension: 256)
+        defer { try? FileManager.default.removeItem(at: outputURL) }
+
+        let data = try Data(contentsOf: outputURL)
+
+        #expect(outputURL.pathExtension.lowercased() == "jpg")
+        #expect(data.starts(with: Data([0xFF, 0xD8])))
+        #expect(data.count > 0)
+    }
+}