Fix crash reading MetaAccountSecrets for accounts predating Tron/Bitcoin/Solana

Found on a real device: opening the backup/export screen for an
existing wallet crashed with
java.lang.IndexOutOfBoundsException: Cannot read 412 of 412
  at .../scale/dataType/optional.read
  at .../scale/Schema.read (x3)
  at SecretStoreV2$getMetaAccountSecrets$2.invokeSuspend

Root cause: MetaAccountSecrets is a positional (SCALE) binary schema.
optional<T>.read() unconditionally reads a presence-flag byte with no
EOF handling - reading a blob that predates a trailing field (written
before that field existed) throws instead of yielding null. This is
also why Solana's own address was never getting backfilled: the
migration's own read of the account's stored secrets was throwing on
every attempted run, silently caught by its own per-account try/catch.

Confirmed via substrate-sdk-android's actual source (Dsl.kt/Delegate.kt)
that Tron/Bitcoin's identical schema-growth pattern carried the exact
same latent risk - it likely only avoided crashing so far by chance
byte-alignment in previously-stored blobs, not because the format
tolerates it.

Fix: read Tron/Bitcoin/Solana's keypair/derivationPath fields (every
field added after the original Substrate+Ethereum schema) through a
SafeOptional DataType that catches a failed read and returns null,
via the library's own custom() field-declaration escape hatch. Write
behavior is byte-for-byte unchanged - only reading a shorter/older
blob now correctly yields null for fields it doesn't have, instead of
crashing every caller of getMetaAccountSecrets (balance sync, signing,
backup/export, this Solana backfill migration itself).

common/src/main/java/io/novafoundation/nova/common/data/secrets/v2/MetaAccountSecrets.kt

@@ -1,11 +1,18 @@
 package io.novafoundation.nova.common.data.secrets.v2
 
+import io.emeraldpay.polkaj.scale.ScaleCodecReader
+import io.emeraldpay.polkaj.scale.ScaleCodecWriter
 import io.novafoundation.nova.common.utils.invoke
 import io.novasama.substrate_sdk_android.encrypt.keypair.Keypair
 import io.novasama.substrate_sdk_android.encrypt.keypair.substrate.Sr25519Keypair
 import io.novasama.substrate_sdk_android.scale.EncodableStruct
 import io.novasama.substrate_sdk_android.scale.Schema
 import io.novasama.substrate_sdk_android.scale.byteArray
+import io.novasama.substrate_sdk_android.scale.custom
+import io.novasama.substrate_sdk_android.scale.dataType.DataType
+import io.novasama.substrate_sdk_android.scale.dataType.optional
+import io.novasama.substrate_sdk_android.scale.dataType.scalable
+import io.novasama.substrate_sdk_android.scale.dataType.string as stringDataType
 import io.novasama.substrate_sdk_android.scale.schema
 import io.novasama.substrate_sdk_android.scale.string
 
@@ -16,6 +23,33 @@ object KeyPairSchema : Schema<KeyPairSchema>() {
     val Nonce by byteArray().optional()
 }
 
+/**
+ * [io.novasama.substrate_sdk_android.scale.dataType.optional]'ın `read()`'i, arabellek (stored blob) bu alan
+ * eklenmeden ÖNCE yazılmış eski verilerde tükendiğinde - yani alan gerçekten "yok" demek istediğinde - zarifçe
+ * null dönmek yerine çöküyor (`reader.readBoolean()` EOF'ta istisna fırlatıyor, yakalama yok). Bu, Tron/Bitcoin
+ * eklenirken de aynı riskti; muhtemelen şans eseri (byte hizalaması tesadüfen tutmuş) hiç yüzeye çıkmamıştı -
+ * Solana'nın 2 yeni alanı arabelleği gerçekten aştırdı ve gerçek cihazda "Cannot read 412 of 412" ile çöktü
+ * (secrets okunan HER yerde: bakiye senkronu, backup ekranı, vs).
+ *
+ * Yazma tarafı [optional]'ın kendisiyle byte-byte aynı kalır - sadece okuma, bu alanın zaten taşıması gereken
+ * anlamı (eskiden hiç yazılmamışsa = yok = null) gerçekten sağlar, kütüphanenin eksik bıraktığı yerde.
+ */
+private class SafeOptional<T>(private val dataType: DataType<T>) : DataType<T?>() {
+    override fun read(reader: ScaleCodecReader): T? {
+        return try {
+            optional(dataType).read(reader)
+        } catch (e: Exception) {
+            null
+        }
+    }
+
+    override fun write(writer: ScaleCodecWriter, value: T?) {
+        optional(dataType).write(writer, value)
+    }
+
+    override fun conformsType(value: Any?) = value == null || dataType.conformsType(value)
+}
+
 object MetaAccountSecrets : Schema<MetaAccountSecrets>() {
     val Entropy by byteArray().optional()
     val SubstrateSeed by byteArray().optional()
@@ -26,14 +60,18 @@ object MetaAccountSecrets : Schema<MetaAccountSecrets>() {
     val EthereumKeypair by schema(KeyPairSchema).optional()
     val EthereumDerivationPath by string().optional()
 
-    val TronKeypair by schema(KeyPairSchema).optional()
-    val TronDerivationPath by string().optional()
+    // Tron/Bitcoin/Solana are every chain-family added after the original Substrate+Ethereum schema - each is a
+    // trailing addition an old, pre-existing stored blob may simply not have any bytes for at all. custom()
+    // + SafeOptional (not the bare `.optional()` used above) makes reading such a blob correctly yield null for
+    // all of them, instead of crashing on the first one whose bytes don't exist - see SafeOptional's doc.
+    val TronKeypair by custom(SafeOptional(scalable(KeyPairSchema)))
+    val TronDerivationPath by custom(SafeOptional(stringDataType))
 
-    val BitcoinKeypair by schema(KeyPairSchema).optional()
-    val BitcoinDerivationPath by string().optional()
+    val BitcoinKeypair by custom(SafeOptional(scalable(KeyPairSchema)))
+    val BitcoinDerivationPath by custom(SafeOptional(stringDataType))
 
-    val SolanaKeypair by schema(KeyPairSchema).optional()
-    val SolanaDerivationPath by string().optional()
+    val SolanaKeypair by custom(SafeOptional(scalable(KeyPairSchema)))
+    val SolanaDerivationPath by custom(SafeOptional(stringDataType))
 }
 
 object ChainAccountSecrets : Schema<ChainAccountSecrets>() {