fix: improve mock Forgejo server port handling

tests/mock-forgejo.py

@@ -698,11 +698,31 @@ class ThreadingHTTPServer(ThreadingMixIn, HTTPServer):
     allow_reuse_address = True
 
 
+def find_free_port(start_port):
+    """Find a free port starting from start_port."""
+    for port in range(start_port, start_port + 100):
+        try:
+            s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+            s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+            s.bind(("0.0.0.0", port))
+            s.close()
+            return port
+        except OSError:
+            continue
+    return start_port
+
+
 def main():
     """Start the mock server."""
     global SHUTDOWN_REQUESTED
 
-    port = int(os.environ.get("MOCK_FORGE_PORT", 3000))
+    base_port = int(os.environ.get("MOCK_FORGE_PORT", 3000))
+    port = find_free_port(base_port)
+
+    # If we found a different port, update the environment variable
+    if port != base_port:
+        os.environ["MOCK_FORGE_PORT"] = str(port)
+
     server = ThreadingHTTPServer(("0.0.0.0", port), ForgejoHandler)
 
     print(f"Mock Forgejo server starting on port {port}", file=sys.stderr)