Expose the credit card merchant as a separate field

[jwtue]

Follow-up to #574. GetCreditCardStatement exposes the Verwendungszweck only through
getPurpose(), which joins all lines into a single string: the merchant name together with the
merchant location and the masked card number, e.g. EXAMPLE SHOP BERLIN 555500******2233.

That string is a poor counterparty name. The same merchant yields a different purpose per location
and per card, so anything that groups by counterparty — for instance naming an expense account in a
bookkeeping tool — fragments one merchant into many.

$statement = $getTransactions->getStatement();
foreach ($statement->getTransactions() as $tx) {
    $tx->getMerchant();      // "EXAMPLE SHOP"
    $tx->getPurpose();       // "EXAMPLE SHOP BERLIN 555500******2233"
    $tx->getPurposeLines();  // ["EXAMPLE SHOP", "BERLIN 555500******2233"]
}

Changes:

• Keep the individual Verwendungszweck lines on CreditCardTransaction instead of discarding them
  after joining
• Add getPurposeLines() (the lines) and getMerchant() (the first line, usually the merchant)
• getPurpose() is unchanged, so this is backwards compatible

getMerchant() returns the first line verbatim. For bookings without a merchant, such as the monthly
settlement, that is the descriptive text the bank places there (e.g. Ausgleich Kreditkartenabrechnung); it is null only when the record carries no Verwendungszweck at all. The
existing testMapsToModel fixture is extended to cover both the merchant and the settlement case.

Developed with the assistance of Claude Code. The field semantics above were checked against the
DKKKU test fixtures introduced in #574, which mirror real BW-Bank/LBBW responses.