Add test for INSTA_UPDATE=unseen mode behavior

The bug affected not just messages but also the core behavior of
INSTA_UPDATE=unseen mode (used by --accept-unseen). With the inverted
logic, the behavior was completely backwards:

Before fix (broken):
- Existing snapshots → incorrectly created .snap.new files
- New snapshots → incorrectly updated in-place

After fix (correct):
- Existing snapshots → update in-place ✓
- New snapshots → create .snap.new files for review ✓

This test verifies the fix correctly handles the unseen detection for
both update behavior logic and display messages.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: max-sixty

cargo-insta/tests/functional/main.rs

@@ -977,3 +977,151 @@ fn test_new_snapshot() {
         "Should not show 'updated snapshot' for new snapshot, got:\n{stderr}"
     );
 }
+
+#[test]
+fn test_unseen_mode_behavior() {
+    use std::process::Stdio;
+
+    // Test that INSTA_UPDATE=unseen correctly distinguishes between existing and new snapshots
+    // This mode is used by --accept-unseen and should:
+    // - Update existing snapshots in-place
+    // - Create .snap.new files for new snapshots
+
+    // Test 1: Existing snapshot should be updated in-place with INSTA_UPDATE=unseen
+    let test_existing = TestFiles::new()
+        .add_file(
+            "Cargo.toml",
+            r#"
+[package]
+name = "test_unseen_mode_existing"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+insta = { path = '$PROJECT_PATH' }
+"#
+            .to_string(),
+        )
+        .add_file(
+            "src/lib.rs",
+            r#"
+#[test]
+fn test_existing() {
+    insta::assert_snapshot!("existing", "Updated value");
+}
+"#
+            .to_string(),
+        )
+        .add_file(
+            "src/snapshots/test_unseen_mode_existing__existing.snap",
+            r#"---
+source: src/lib.rs
+expression: "\"Old value\""
+---
+Old value
+"#
+            .to_string(),
+        )
+        .create_project();
+
+    let output = test_existing
+        .insta_cmd()
+        .env("INSTA_UPDATE", "unseen")
+        .args(["test", "--", "--nocapture"])
+        .stderr(Stdio::piped())
+        .output()
+        .unwrap();
+
+    assert!(
+        output.status.success(),
+        "Command should succeed\nstderr: {}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("updated snapshot"),
+        "Expected 'updated snapshot' message for existing snapshot with INSTA_UPDATE=unseen, got:\n{stderr}"
+    );
+
+    // Verify the snapshot was updated in-place, not as a .new file
+    let snapshot_path = test_existing
+        .workspace_dir
+        .join("src/snapshots/test_unseen_mode_existing__existing.snap");
+    let snapshot_content = fs::read_to_string(&snapshot_path).unwrap();
+    assert!(
+        snapshot_content.contains("Updated value"),
+        "Existing snapshot should be updated in-place with INSTA_UPDATE=unseen"
+    );
+    let new_file_path = test_existing
+        .workspace_dir
+        .join("src/snapshots/test_unseen_mode_existing__existing.snap.new");
+    assert!(
+        !new_file_path.exists(),
+        "Should not create .snap.new file for existing snapshot with INSTA_UPDATE=unseen"
+    );
+
+    // Test 2: New snapshot should create .snap.new file with INSTA_UPDATE=unseen
+    let test_new = TestFiles::new()
+        .add_file(
+            "Cargo.toml",
+            r#"
+[package]
+name = "test_unseen_mode_new"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+insta = { path = '$PROJECT_PATH' }
+"#
+            .to_string(),
+        )
+        .add_file(
+            "src/lib.rs",
+            r#"
+#[test]
+fn test_new() {
+    insta::assert_snapshot!("new", "New value");
+}
+"#
+            .to_string(),
+        )
+        .create_project();
+
+    let output = test_new
+        .insta_cmd()
+        .env("INSTA_UPDATE", "unseen")
+        .args(["test", "--", "--nocapture"])
+        .stderr(Stdio::piped())
+        .output()
+        .unwrap();
+
+    // The command should fail because there are pending snapshots to review
+    assert!(
+        !output.status.success(),
+        "Command should fail with pending snapshots\nstderr: {}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("stored new snapshot"),
+        "Expected 'stored new snapshot' message for new snapshot with INSTA_UPDATE=unseen, got:\n{stderr}"
+    );
+
+    // Verify a .snap.new file was created for the new snapshot
+    let new_file_path = test_new
+        .workspace_dir
+        .join("src/snapshots/test_unseen_mode_new__new.snap.new");
+    assert!(
+        new_file_path.exists(),
+        "Should create .snap.new file for new snapshot with INSTA_UPDATE=unseen"
+    );
+    let snapshot_path = test_new
+        .workspace_dir
+        .join("src/snapshots/test_unseen_mode_new__new.snap");
+    assert!(
+        !snapshot_path.exists(),
+        "Should not create .snap file directly for new snapshot with INSTA_UPDATE=unseen"
+    );
+}