Add DocumentState tests for file change detection and reload behavior

Cover the file monitor's change detection pipeline that the auto-raise
feature depends on: file modification sets fileChanged, no-modification
leaves it unset, loadFile resets it, and reload guards against nil URL.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: myrison

Tests/MarkdownViewerTests/DocumentStateTests.swift

@@ -0,0 +1,113 @@
+import XCTest
+@testable import MarkdownViewer
+
+final class DocumentStateTests: XCTestCase {
+
+    // MARK: - File Change Detection
+    //
+    // These tests verify that DocumentState's file monitor correctly
+    // detects external file modifications and surfaces them via the
+    // published fileChanged flag.  The auto-raise feature (added in
+    // this PR) depends on this detection path to decide whether to
+    // bring the window to front.
+
+    func testFileChangedSetToTrueWhenMonitoredFileIsModified() throws {
+        let file = try TemporaryFile.create(named: "test.md")
+        defer { file.cleanup() }
+
+        try "# Hello".write(to: file.url, atomically: true, encoding: .utf8)
+
+        let state = DocumentState()
+        state.loadFile(at: file.url)
+
+        XCTAssertFalse(state.fileChanged, "fileChanged should be false immediately after load")
+
+        // Modify the file after a brief delay so the modification date differs
+        let expectation = expectation(description: "File change detected")
+        DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
+            try? "# Updated".write(to: file.url, atomically: true, encoding: .utf8)
+        }
+
+        // Give the file monitor time to detect the change
+        DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {
+            expectation.fulfill()
+        }
+
+        wait(for: [expectation], timeout: 5.0)
+        XCTAssertTrue(state.fileChanged, "fileChanged should be true after file modification")
+    }
+
+    func testFileChangedNotSetWhenFileIsNotModified() throws {
+        let file = try TemporaryFile.create(named: "stable.md")
+        defer { file.cleanup() }
+
+        try "# Stable".write(to: file.url, atomically: true, encoding: .utf8)
+
+        let state = DocumentState()
+        state.loadFile(at: file.url)
+
+        let expectation = expectation(description: "Wait period")
+        DispatchQueue.main.asyncAfter(deadline: .now() + 1.0) {
+            expectation.fulfill()
+        }
+
+        wait(for: [expectation], timeout: 3.0)
+        XCTAssertFalse(state.fileChanged, "fileChanged should remain false when file is not modified")
+    }
+
+    func testLoadFileResetsFileChangedFlag() throws {
+        let file = try TemporaryFile.create(named: "reload.md")
+        defer { file.cleanup() }
+
+        try "# First".write(to: file.url, atomically: true, encoding: .utf8)
+
+        let state = DocumentState()
+        state.loadFile(at: file.url)
+
+        // Trigger a file change
+        let changeExpectation = expectation(description: "File change detected")
+        DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
+            try? "# Modified".write(to: file.url, atomically: true, encoding: .utf8)
+        }
+        DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {
+            changeExpectation.fulfill()
+        }
+        wait(for: [changeExpectation], timeout: 5.0)
+        XCTAssertTrue(state.fileChanged)
+
+        // Reload should reset the flag
+        state.loadFile(at: file.url)
+        XCTAssertFalse(state.fileChanged, "loadFile should reset fileChanged to false")
+    }
+
+    // MARK: - Reload Behavior
+
+    func testReloadDoesNothingWhenNoFileIsLoaded() {
+        let state = DocumentState()
+        XCTAssertNil(state.currentURL)
+        let previousToken = state.reloadToken
+
+        state.reload()
+
+        // reloadToken should not change when there is no file to reload
+        XCTAssertEqual(state.reloadToken, previousToken,
+                       "reload() with no currentURL should not generate a new reloadToken")
+    }
+
+    func testReloadUpdatesReloadToken() throws {
+        let file = try TemporaryFile.create(named: "doc.md")
+        defer { file.cleanup() }
+
+        try "# Doc".write(to: file.url, atomically: true, encoding: .utf8)
+
+        let state = DocumentState()
+        state.loadFile(at: file.url)
+
+        let tokenBefore = state.reloadToken
+        state.reload()
+
+        XCTAssertNotNil(state.reloadToken)
+        XCTAssertNotEqual(state.reloadToken, tokenBefore,
+                          "reload() should produce a new reloadToken")
+    }
+}