Merge bitcoin/bitcoin#28067: descriptors: do not return top-level only funcs as sub descriptors

dd9633b516 test: wallet, add coverage for watch-only raw sh script migration (furszy)
cc781a2180 descriptor: InferScript, do not return top-level only func as sub descriptor (furszy)
286e0c7d5e wallet: loading, log descriptor parsing error details (furszy)

Pull request description:

  Linked to #28057.

  Currently, the `InferScript` function returns an invalid descriptor when it tries to infer a p2sh-p2pkh script whose pubkey is not known by the wallet.

  This behavior occurs because the inference process bypasses the `pkh` subscript when the pubkey is not contained by the wallet (no pubkey provider), interpreting it as a `sh(addr(ADDR))` descriptor. Then, the failure arises because the `addr()` function is restricted to being used only at the top level.

  For reviewers, would recommend to start by examining the functional test to understand the context and the circumstances on which this can result in a fatal error (e.g. during the migration process).

ACKs for top commit:
  achow101:
    ACK dd9633b516
  darosior:
    utACK dd9633b516

Tree-SHA512: 61e763206c604c372019d2c36e31684f3dddf81f8b154eb9aba5cd66d8d61bda457ed4e591613eb6ce6c76cf7c3f11764abc6cd727a7c2b6414f1065783be032

src/script/descriptor.cpp

@@ -1723,6 +1723,10 @@ std::unique_ptr<DescriptorImpl> InferScript(const CScript& script, ParseScriptCo
         }
     }
 
+    // The following descriptors are all top-level only descriptors.
+    // So if we are not at the top level, return early.
+    if (ctx != ParseScriptContext::TOP) return nullptr;
+
     CTxDestination dest;
     if (ExtractDestination(script, dest)) {
         if (GetScriptForDestination(dest) == script) {