Use the node's admin password for the migration bridge AP

The Wi-Fi, LoRa first-stage and Partition Expander bridges serve an
unauthenticated /update behind an AP whose password was fixed and published,
so anyone in range during a migration could upload firmware while the private
key is staged. Use the old node's saved admin password as the WPA2
passphrase, read from /com_prefs or /node_prefs at offset 56 as the firmware
loads it. The operator already used that password to start the old updater.

Fall back to the published password, with a serial warning, when the saved
admin password is missing, unreadable, shorter than 8 characters or not
printable ASCII, so no node becomes unreachable. The legacy filesystem is
mounted without formatting and the password is never printed. Migration
admission, flash sequencing, /update gating and the USB bridge are unchanged.
Update both guides and the package README.

Add a sanitizer-backed host test that compiles the bridge's actual Wi-Fi
block against fake SPIFFS and WiFi: both boots, file precedence, WPA2 length
and terminator bounds, printable characters, stock 1.10/1.11 file sizes,
truncated fields, and mount, seek and partial-read failures. It fails 8 of 8
on the previous code and catches 27 of 27 code mutations. Verified 13 related
host test files and the esp32_s3_4mb_partition_migrator build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/run-unit-tests.yml

@@ -774,6 +774,7 @@ jobs:
           python3 -B test/test_firmware_capabilities.py
           MESHCORE_TEST_REAL_PIO_PATHS=1 python3 -B test/test_esp32_partition_migration_build_dir.py -v
           python3 -B test/test_esp32_partition_migration_usb.py -v
+          python3 -B test/test_esp32_partition_migrator_ap_password.py -v
           python3 -B test/test_waveshare_partition_migration.py -v
           python3 -B test/test_usb_partition_bridge_set.py -v
           bash test/test_build_profiles.sh