Keep whole contact records ahead of a partial trailing record

Upstream MeshCore, and this fork up to v1.17.1.6 except ESP32 Full
builds, rewrote /contacts3 in place, one record after another. A reset
or a full volume during that save leaves whole records followed by part
of the next one. Upstream reads such a file up to the short read. Here
readableContactFile() treats any size that is not a multiple of 152
bytes as unreadable, and since 2311160a9 the bounded recovery then
deletes /contacts3. On the first boot after an upgrade, three whole
records plus one stray byte became zero contacts.

Invariant: on ESP32, RP2040 and STM32, when /contacts3 holds at least
one whole record followed by part of one, and /contacts3.bak does not
exist, the whole records load and nothing is written or deleted at
boot. The next ordinary save publishes only whole records through the
existing transaction. Unchanged: whole-record images; a backup still
wins over an incomplete primary, including a backup that is unreadable
or of unknown presence at boot, which keeps its retry at the recovery
boundary; read errors keep the bounded recovery; a file with no whole
record is still discarded; nRF52 is not touched.

selectContactFile() makes this check only after the strict checks of
both names fail, through a new partial_tail mode of
readableContactFile(). loadContacts() drops its own size check; its
whole-record loop bound already stops before the tail.

Add test_companion_contact_partial_tail.py, which runs the real load,
recovery and save paths on the uncached ESP32, RP2040 and STM32
harness and on the ESP32 Full cache harness under ASan/UBSan. On the
old code all four fail with zero contacts and /contacts3 deleted. It
catches 14 of 15 mutations of this change, including a full revert;
the other drops a guard whose value is read only when the file handle
is valid. Two cases in the uncached storage fixture used a one-byte
truncation as their unreadable image; they now use a file shorter than
one record, so they still exercise the discard path.

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

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

@@ -411,6 +411,7 @@ jobs:
           python3 -B test/test_companion_identity_cli.py -v
           python3 -B test/test_companion_packet_stream.py -v
           python3 -B test/test_companion_uncached_storage.py -v
+          python3 -B test/test_companion_contact_partial_tail.py -v
           python3 -B test/test_companion_channel_storage.py -v
           python3 -B test/test_nrf52_extrafs_contract.py -v
           python3 -B test/test_companion_ota_config.py -v