test: add property-based invariant test for single browser session

Author: skulidropek

tests/browser_invariants.rs

@@ -0,0 +1,18 @@
+//! Property-based tests for the single-browser invariant
+//!
+//! INVARIANT (theorem): ∀ project_id, repeated calls to start_browser yield exactly one container.
+
+use docker_git_browser_connection::BrowserConnection;
+use proptest::prelude::*;
+
+proptest! {
+    #[test]
+    fn single_browser_invariant(project_id in "[a-z0-9-]{1,20}") {
+        // Note: this is illustrative; real test would require Docker test harness
+        // In CI we use #[ignore] or testcontainers
+        let conn = BrowserConnection::new().unwrap();
+        let url1 = conn.get_cdp_url(&project_id);
+        let url2 = conn.get_cdp_url(&project_id);
+        prop_assert!(conn.is_single_browser_session(&url1, &url2));
+    }
+}