fix(nvs_flash): zeroize XTS contexts when encrypted partition is destroyed

NVSEncryptedPartition held two XTS_CONTEXT members (mEctxt, mDctxt) for
encryption / decryption. Their AES round keys are derived from the NVS
encryption key (HMAC-derived or plaintext from nvs_keys partition) and
therefore are sensitive secrets.

The destructor was empty, so when the NVS encrypted partition object
was destroyed -- on nvs_flash_deinit_partition(), on initialization
errors, and on any other teardown path -- the XTS round keys were left
in DRAM until the freed object's memory happened to be overwritten by
a later allocation. A subsequent stack/heap leak primitive would
recover the AES key from those bytes.

Fix:
* Initialize both XTS contexts in the constructor so the destructor's
  free path is always safe (previously xts_init was only called in
  init(); destruction before init() would have run xts_free on an
  uninitialized struct).
* Provide a real destructor that calls XTS_FUNC(xts_free) on both
  contexts, which performs mbedtls_platform_zeroize / esp_aes_xts free
  semantics on the underlying AES contexts.

components/nvs_flash/src/nvs_encrypted_partition.cpp

@@ -13,12 +13,28 @@ namespace nvs {
 
 #ifdef CONFIG_NVS_BDL_STACK
 NVSEncryptedPartition::NVSEncryptedPartition(const char* label, const esp_blockdev_handle_t bdl, const bool managed_bdl)
-    : NVSPartition(label, bdl, managed_bdl) { }
+    : NVSPartition(label, bdl, managed_bdl)
+{
+    XTS_FUNC(xts_init)(&mEctxt);
+    XTS_FUNC(xts_init)(&mDctxt);
+}
 #else
 NVSEncryptedPartition::NVSEncryptedPartition(const esp_partition_t *partition)
-    : NVSPartition(partition) { }
+    : NVSPartition(partition)
+{
+    XTS_FUNC(xts_init)(&mEctxt);
+    XTS_FUNC(xts_init)(&mDctxt);
+}
 #endif // CONFIG_NVS_BDL_STACK
 
+NVSEncryptedPartition::~NVSEncryptedPartition()
+{
+    /* Wipe AES round keys derived from the NVS encryption key so they are not
+     * left in DRAM after the partition object is destroyed. */
+    XTS_FUNC(xts_free)(&mEctxt);
+    XTS_FUNC(xts_free)(&mDctxt);
+}
+
 esp_err_t NVSEncryptedPartition::init(nvs_sec_cfg_t* cfg)
 {
     uint8_t* eky = reinterpret_cast<uint8_t*>(cfg);

components/nvs_flash/src/nvs_encrypted_partition.hpp

@@ -23,7 +23,7 @@ public:
     NVSEncryptedPartition(const esp_partition_t *partition);
 #endif
 
-    virtual ~NVSEncryptedPartition() { }
+    virtual ~NVSEncryptedPartition();
 
     /**
      * Initializes the AES encryption components with the provided configuration.