lua: fix deployed MCU update runtime path

[cpunt]

Summary

• fix deployed UART Fabric/HAL contract wiring
• configure UI admin auth from runtime env
• provide default UI JSON response encoding
• ignore self-echoed and wrong-peer Fabric handshakes

Tests

• TEST_FILTER='unit.fabric.test_session' make test
• TEST_FILTER='unit.fabric.test_link' make test
• TEST_FILTER='mcu_update_http_uart_spec' make test
• git diff --check

Notes

• Temporary CM5 UART trace logging and the flush_each config experiment are not included.
• Hardware status report is intentionally kept out of the PR and will be shared separately.