Merge bitcoin/bitcoin#31423: wallet: migration, avoid creating spendable wallet from a watch-only legacy wallet

b789907346 wallet: migration, avoid creating spendable wallet from a watch-only legacy wallet (furszy)
e86d71b749 wallet: refactor, dedup wallet re-loading code (furszy)
1de423e0a0 wallet: introduce method to return all db created files (furszy)
d04f6a97ba refactor: remove sqlite dir path back-and-forth conversion (furszy)

Pull request description:

  Currently, the migration process creates a brand-new descriptor wallet with no
  connection to the user's legacy wallet when the legacy wallet lacks key material
  and contains only watch-only scripts. This behavior is not aligned with user
  expectations. If the legacy wallet contains only watch-only scripts, the migration
  process should only generate a watch-only wallet instead.

  TODO List:
  * Explain that `migratewallet` renames the watch-only after migration, and
  also that the wallet will not have keys enabled.

ACKs for top commit:
  achow101:
    ACK b789907346
  pablomartin4btc:
    tACK b789907346
  rkrux:
    LGTM ACK b789907346

Tree-SHA512: 1d583ac4b206fb477e9727daf4b5ad9c3e18b12d40e1ab4a61e8565da44c3d0327c892b51cf47b4894405d122e414cefb6b6366c357e02a74a7ca96e06762d83

src/wallet/test/util.h

@@ -109,6 +109,7 @@ public:
     void Close() override {}
 
     std::string Filename() override { return "mockable"; }
+    std::vector<fs::path> Files() override { return {}; }
     std::string Format() override { return "mock"; }
     std::unique_ptr<DatabaseBatch> MakeBatch() override { return std::make_unique<MockableBatch>(m_records, m_pass); }
 };