desc spkm: Return SigningProvider only if we have the privkey

If we know about a pubkey that's in our descriptor, but we don't have
the private key, don't return a SigningProvider for that pubkey.

This is specifically an issue for Taproot outputs that use the H point
as the resulting PSBTs may end up containing irrelevant information
because the H point was detected as a pubkey each unrelated descriptor
knew about.

src/script/signingprovider.cpp

@@ -62,6 +62,11 @@ bool FlatSigningProvider::GetKeyOrigin(const CKeyID& keyid, KeyOriginInfo& info)
     if (ret) info = std::move(out.second);
     return ret;
 }
+bool FlatSigningProvider::HaveKey(const CKeyID &keyid) const
+{
+    CKey key;
+    return LookupHelper(keys, keyid, key);
+}
 bool FlatSigningProvider::GetKey(const CKeyID& keyid, CKey& key) const { return LookupHelper(keys, keyid, key); }
 bool FlatSigningProvider::GetTaprootSpendData(const XOnlyPubKey& output_key, TaprootSpendData& spenddata) const
 {

src/script/signingprovider.h

@@ -215,6 +215,7 @@ struct FlatSigningProvider final : public SigningProvider
     bool GetCScript(const CScriptID& scriptid, CScript& script) const override;
     bool GetPubKey(const CKeyID& keyid, CPubKey& pubkey) const override;
     bool GetKeyOrigin(const CKeyID& keyid, KeyOriginInfo& info) const override;
+    bool HaveKey(const CKeyID &keyid) const override;
     bool GetKey(const CKeyID& keyid, CKey& key) const override;
     bool GetTaprootSpendData(const XOnlyPubKey& output_key, TaprootSpendData& spenddata) const override;
     bool GetTaprootBuilder(const XOnlyPubKey& output_key, TaprootBuilder& builder) const override;

src/wallet/scriptpubkeyman.cpp

@@ -2456,7 +2456,11 @@ std::unique_ptr<FlatSigningProvider> DescriptorScriptPubKeyMan::GetSigningProvid
     int32_t index = it->second;
 
     // Always try to get the signing provider with private keys. This function should only be called during signing anyways
-    return GetSigningProvider(index, true);
+    std::unique_ptr<FlatSigningProvider> out = GetSigningProvider(index, true);
+    if (!out->HaveKey(pubkey.GetID())) {
+        return nullptr;
+    }
+    return out;
 }
 
 std::unique_ptr<FlatSigningProvider> DescriptorScriptPubKeyMan::GetSigningProvider(int32_t index, bool include_private) const