[BT] Default BLE AES key empty, skip decryption without a key (#2310)

* [BT] Default BLE AES key empty, skip decryption without a key

Shipping a non-empty default key caused fresh installs to attempt
decryption of every encrypted PVVX/BTHome v2/Victron advertisement with
the placeholder string from User_config.h. Frames that didn't happen to
share that key were silently dropped at the CCM auth step, masking
otherwise-decodable devices (notably Victron MPPT) until users found and
entered a per-MAC key.

Initialise ble_aes[] to "" and treat an empty default as "no default
configured" in the decryption path - mirroring Theengs Gateway, which
has no default and skips frames whose MAC has no bindkey. Per-MAC keys
in ble_aes_keys still take precedence; the BLE_AES macro remains as the
WebUI placeholder hint.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* [BT] clang-format: move BLE_AES default comment above #define

clang-format-9 rejected the trailing-comment continuation on the empty
BLE_AES default line. Move the explanation to a leading // block above
the #ifndef so the directive line is comment-free.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* [BT] Use THEENGS_LOG_TRACE for missing-AES-key skip log

Previous commit referenced THEENGS_LOG_DEBUG, which OMG does not define
(only VERBOSE/TRACE/NOTICE/WARNING/ERROR exist), breaking every BLE
firmware build. Switch to THEENGS_LOG_TRACE — same level as the adjacent
"Custom AES key" / "Default AES key" trace lines.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

main/main.cpp

@@ -96,6 +96,10 @@ char gateway_name[parameters_size + 1] = Gateway_Name;
 unsigned long lastDiscovery = 0;
 
 #if BLEDecryptor
+// BLE_AES defaults to "" (see User_config.h) so encrypted PVVX/BTHome/Victron
+// frames are only decrypted when a per-MAC key is configured (ble_aes_keys) or
+// the user enters a default. Sites shipping a stock default key can still set
+// it at build time via -DBLE_AES='"<32-hex-key>"'.
 char ble_aes[parameters_size] = BLE_AES;
 StaticJsonDocument<JSON_BLE_AES_CUSTOM_KEYS> ble_aes_keys;
 #endif