The word finder needs to account for repeated letters and blank tiles in a rack. Implement matching so each tile can be spent only once and blanks cover only the missing letters.
Build the implementation in Python 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 rack containing lowercase ASCII letters and ? blanks, and words containing nonempty lowercase ASCII strings. Each ? supplies any one letter. Evaluate words independently using the whole rack each time. Return unique formable words sorted lexicographically; unused rack tiles are allowed. Reject invalid rack or word characters.
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.