A user reported to me that they are unable to create a PSBT with a legacy watchonly wallet when they use the GUI. They get an error "The amount exceeds your balance".

I think the issue is that the GUI will retrieve the wallet's balance via `AvailableCoins` before calling `CreateTransaction`. This call to `AvailableCoins` is resulting in fewer coins being returned with recent changes to how `AvailableCoins` works, particularly with additional filtering of spendable coins and preset coins.

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Note that this does not effect descriptor wallets. A workaround for this issue is to switch to using descriptor wallets. An existing legacy wallet can be turned into a descriptor wallet using the `migratewallet` RPC. However many watchonly legacy wallets are likely the result of importing descriptors via `importmulti`. For such wallets, I recommend creating a new descriptor wallet and re-importing the descriptors.
void setBalance(const interfaces::WalletBalances& balances);
Q_SIGNALS:
    void coinsSent(const uint256& txid);
private:
    Ui::SendCoinsDialog *ui;
    ClientModel* clientModel{nullptr};
    WalletModel* model{nullptr};
    std::unique_ptr<wallet::CCoinControl> m_coin_control;
    std::unique_ptr<WalletModelTransaction> m_current_transaction;
    bool fNewRecipientAllowed{true};
    bool fFeeMinimized{true};
    const PlatformStyle *platformStyle;
    
    void presentPSBT(PartiallySignedTransaction& psbt);
    
    
    
    void processSendCoinsReturn(const WalletModel::SendCoinsReturn &sendCoinsReturn, const QString &msgArg = QString());
    void minimizeFeeSection(bool fMinimize);
    
    bool PrepareSendText(QString& question_string, QString& informative_text, QString& detailed_text);
    /* Sign PSBT using external signer.
     *
     * @param[in,out] psbtx the PSBT to sign
     * @param[in,out] mtx needed to attempt to finalize
     * @param[in,out] complete whether the PSBT is complete (a successfully signed multisig transaction may not be complete)
     *
     * @returns false if any failure occurred, which may include the user rejection of a transaction on the device.
     */
    bool signWithExternalSigner(PartiallySignedTransaction& psbt, CMutableTransaction& mtx, bool& complete);