The path finder must account for paired portals when finding the shortest route across the maze. Finish navigation so teleportation and ordinary movement produce the required distance or unreachable result.
Build the implementation in Go under /app, with /app/run.sh as its entry point. Read one JSON request from standard input and write one JSON result to standard output. Input has grid as equal-length strings containing # walls, . floor, one S, one E and optional uppercase portal letters other than S/E. Each portal letter must occur exactly twice. A move to an adjacent nonwall cell costs one; entering a portal immediately moves to its partner at no extra cost, without bouncing back. Return shortest moves to E or null. Invalid grids must be rejected.
Reject inputs that violate an explicit validation rule with exit status 2, no standard output, and {"error":"invalid_input"} on standard error. For valid input, exit with status 0. Keep the implementation reusable for new inputs; it must work offline and must not depend on the example data being the only data it receives.
Repair or replace the unfinished /app/run.sh and add source files under /app. Malformed JSON must use the same invalid-input response. Do not read /tests or /oracle from the application.