wallet: disallow migration of invalid or not-watched scripts

The legacy wallet allowed to import any raw script, without checking if
it was valid or not. Appending it to the watch-only set.

This causes a crash in the migration process because we are only
expecting to find valid scripts inside the legacy spkm.

These stored scripts internally map to `ISMINE_NO` (same as if they
weren't stored at all..).

So we need to check for these special case, and take into account that
the legacy spkm could be storing invalid not watched scripts.

Which, in code words, means IsMineInner() returning IsMineResult::INVALID
for them.

src/wallet/scriptpubkeyman.h

@@ -523,6 +523,12 @@ public:
     std::set<CKeyID> GetKeys() const override;
     std::unordered_set<CScript, SaltedSipHasher> GetScriptPubKeys() const override;
 
+    /**
+     * Retrieves scripts that were imported by bugs into the legacy spkm and are
+     * simply invalid, such as a sh(sh(pkh())) script, or not watched.
+     */
+    std::unordered_set<CScript, SaltedSipHasher> GetNotMineScriptPubKeys() const;
+
     /** Get the DescriptorScriptPubKeyMans (with private keys) that have the same scriptPubKeys as this LegacyScriptPubKeyMan.
      * Does not modify this ScriptPubKeyMan. */
     std::optional<MigrationData> MigrateToDescriptor();