Fix linux BLE deadload (#26418)

Make is so ChipDeviceScanner::StartScan is only called while in the Matter Context

Author: tehampson

src/controller/python/chip/ble/LinuxImpl.cpp

@@ -130,7 +130,11 @@ extern "C" void * pychip_ble_start_scanning(PyObject * context, void * adapter,
         return nullptr;
     }
 
-    if (scanner->StartScan(chip::System::Clock::Milliseconds32(timeoutMs)) != CHIP_NO_ERROR)
+    CHIP_ERROR err = CHIP_NO_ERROR;
+    chip::DeviceLayer::PlatformMgr().LockChipStack();
+    err = scanner->StartScan(chip::System::Clock::Milliseconds32(timeoutMs));
+    chip::DeviceLayer::PlatformMgr().UnlockChipStack();
+    if (err != CHIP_NO_ERROR)
     {
         return nullptr;
     }

src/platform/Linux/bluez/ChipDeviceScanner.cpp

@@ -141,6 +141,7 @@ std::unique_ptr<ChipDeviceScanner> ChipDeviceScanner::Create(BluezAdapter1 * ada
 
 CHIP_ERROR ChipDeviceScanner::StartScan(System::Clock::Timeout timeout)
 {
+    assertChipStackLockedByCurrentThread();
     ReturnErrorCodeIf(mIsScanning, CHIP_ERROR_INCORRECT_STATE);
 
     mIsScanning = true; // optimistic, to allow all callbacks to check this
@@ -157,9 +158,7 @@ CHIP_ERROR ChipDeviceScanner::StartScan(System::Clock::Timeout timeout)
         return CHIP_ERROR_INTERNAL;
     }
 
-    DeviceLayer::PlatformMgr().LockChipStack();
     CHIP_ERROR err = chip::DeviceLayer::SystemLayer().StartTimer(timeout, TimerExpiredCallback, static_cast<void *>(this));
-    DeviceLayer::PlatformMgr().UnlockChipStack();
 
     if (err != CHIP_NO_ERROR)
     {

src/platform/Linux/bluez/ChipDeviceScanner.h

@@ -66,6 +66,9 @@ class ChipDeviceScanner
     ~ChipDeviceScanner();
 
     /// Initiate a scan for devices, with the given timeout
+    ///
+    /// This method must be called while in the Matter context (from the Matter event
+    /// loop, or while holding the Matter stack lock).
     CHIP_ERROR StartScan(System::Clock::Timeout timeout);
 
     /// Stop any currently running scan