Stop BLE log streaming from starving the NimBLE notify pool (#11253)

sendLog() notified once per firmware log line with no backpressure. With
debug_log_api_enabled and a subscribed client, a logging burst drains the
msys_1 mbuf pool that every GATT notification and ATT response allocates
from, so ble_gatts_notify_custom starts returning BLE_HS_ENOMEM (rc=6) --
for the fromNum doorbell too, not just the log line that exhausted it.
Each failure then prints an error over serial at ~12ms apiece, which is
itself enough to throttle the main loop during a burst.

Register a callback on the log characteristic so the host's own
ERROR_GATT verdict pauses log notifies for 250ms and lets the pool
refill. Keying off the failure rather than a free-block count keeps this
correct however the pools are sized.

Also restore CONFIG_BT_NIMBLE_MSYS_1_BLOCK_COUNT to the IDF default of
12. Pool selection is by block size and a notify request always resolves
to the smallest pool, so msys_1 is the only pool GATT draws from; 8 was
thin once anything chains or fragments. msys_2 and the ACL/EVT transport
pools are left trimmed -- neither is on the notify path, and restoring
the full defaults would re-spend the contiguous allocation that #10741
was fixing on heap-tight boards.

sendLog() also lacked the null check onNowHasData() already has, so a log
line arriving during BLE teardown dereferenced a freed characteristic.

Fixes #11245

src/nimble/NimbleBluetooth.cpp

@@ -637,6 +637,21 @@ class NimbleBluetoothFromRadioCallback : public BLECharacteristicCallbacks
     }
 };
 
+// One log notify per log line shares the msys_1 mbuf pool with the fromNum doorbell and ATT
+// responses, so a logging burst starves them; back off on rejection until the pool refills.
+static constexpr uint32_t LOG_NOTIFY_BACKOFF_MS = 250;
+static std::atomic<uint32_t> lastLogNotifyFailureMs{0};
+
+class NimbleBluetoothLogRadioCallback : public BLECharacteristicCallbacks
+{
+    void onStatus(BLECharacteristic *, Status s, uint32_t) override
+    {
+        // ERROR_GATT is the only status meaning the host refused it; the rest never allocated.
+        if (s == Status::ERROR_GATT)
+            lastLogNotifyFailureMs.store(millis());
+    }
+};
+
 class NimbleBluetoothSecurityCallback : public BLESecurityCallbacks
 {
     void onPassKeyNotify(uint32_t passkey) override
@@ -1005,6 +1020,9 @@ void NimbleBluetooth::setupService()
     static NimbleBluetoothFromRadioCallback fromRadioCallbacks;
     FromRadioCharacteristic->setCallbacks(&fromRadioCallbacks);
 
+    static NimbleBluetoothLogRadioCallback logRadioCallbacks;
+    logRadioCharacteristic->setCallbacks(&logRadioCallbacks);
+
     bleService->start();
 
     // Setup the battery service
@@ -1056,6 +1074,11 @@ void NimbleBluetooth::sendLog(const uint8_t *logMessage, size_t length)
     if (!isConnected() || length > 512) {
         return;
     }
+    if (!logRadioCharacteristic) // BLE may have been torn down; never notify a freed characteristic
+        return;
+    // Pool still under pressure; drop this line rather than spend a buffer fromNum needs.
+    if (Throttle::isWithinTimespanMs(lastLogNotifyFailureMs.load(), LOG_NOTIFY_BACKOFF_MS))
+        return;
     logRadioCharacteristic->setValue(logMessage, length);
     logRadioCharacteristic->notify();
 }