Merge bitcoin/bitcoin#25634: wallet, tests: Expand and test when the blank wallet flag should be un/set

cdba23db35 wallet: Document blank flag use in descriptor wallets (Ryan Ofsky)
43310200dc wallet: Ensure that the blank wallet flag is unset after imports (Andrew Chow)
e9379f1ffa rpc, wallet: Include information about blank flag (Andrew Chow)

Pull request description:

  The `blank` wallet flag is used to indicate that the wallet intentionally does not have any keys, scripts, or descriptors, and it prevents the automatic generation of those things for such a wallet. Once the wallet contains any of those data, it is unnecessary, and possibly incorrect, to have `blank` set. This PR fixes a few places where this was not properly happening. It also adds a test for this unset behavior.

ACKs for top commit:
  S3RK:
    reACK cdba23db35
  ryanofsky:
    Code review ACK cdba23db35. Only change since last review is dropping the commit which makes createwallet RPC set BLANK flag automatically when DISABLE_PRIVATE_KEYS flag is set

Tree-SHA512: 85bc2a9754df0531575d5c8f4ad7e8f38dcd50083dc29b3283dacf56feae842e81f34654c5e1781f2dadb0560ff80e454bbc8ca3b2d1fab1b236499ae9abd7da

src/wallet/scriptpubkeyman.cpp

@@ -757,12 +757,12 @@ bool LegacyScriptPubKeyMan::AddKeyPubKeyWithDB(WalletBatch& batch, const CKey& s
         RemoveWatchOnly(script);
     }
 
+    m_storage.UnsetBlankWalletFlag(batch);
     if (!m_storage.HasEncryptionKeys()) {
         return batch.WriteKey(pubkey,
                                                  secret.GetPrivKey(),
                                                  mapKeyMetadata[pubkey.GetID()]);
     }
-    m_storage.UnsetBlankWalletFlag(batch);
     return true;
 }