rpc: Use FlatSigningProvider in decodescript

Using a FillableSigningProvider results in decodescript being unable to
infer descriptors for scripts larger than 520 bytes. Using a
FlatSigningProvider resolves this.

Author: achow101

src/rpc/rawtransaction.cpp

@@ -522,14 +522,14 @@ static RPCHelpMan decodescript()
         if (can_wrap_P2WSH) {
             UniValue sr(UniValue::VOBJ);
             CScript segwitScr;
-            FillableSigningProvider provider;
+            FlatSigningProvider provider;
             if (which_type == TxoutType::PUBKEY) {
                 segwitScr = GetScriptForDestination(WitnessV0KeyHash(Hash160(solutions_data[0])));
             } else if (which_type == TxoutType::PUBKEYHASH) {
                 segwitScr = GetScriptForDestination(WitnessV0KeyHash(uint160{solutions_data[0]}));
             } else {
                 // Scripts that are not fit for P2WPKH are encoded as P2WSH.
-                provider.AddCScript(script);
+                provider.scripts[CScriptID(script)] = script;
                 segwitScr = GetScriptForDestination(WitnessV0ScriptHash(script));
             }
             ScriptToUniv(segwitScr, /*out=*/sr, /*include_hex=*/true, /*include_address=*/true, /*provider=*/&provider);