Handle malformed dashboard URL encodings

[w287346141]

Summary

• handle malformed percent-encoding in dashboard file-read paths with a 400 instead of an uncaught exception
• handle malformed percent-encoding in dashboard session names with a 400 instead of an uncaught exception
• add focused regression tests for both decode paths

Validation

• npm.cmd test -- --run tests/dashboard-api-decode.test.ts tests/server-dashboard.test.ts
• npm.cmd run lint
• npm.cmd run typecheck
• git diff --check
• push hook ran npm.cmd run verify