Commerzbank photoTAN: current TAN mechanism remains `999` while 2-step mechanism is `900`

[QHLe]

Describe the bug
With Commerzbank photoTAN, the client can end up in a state where the current TAN mechanism is still 999, while the bank-advertised usable two-step mechanism is 900.

This makes the bootstrap/follow-up flow brittle. In particular, code paths such as minimal_interactive_cli_bootstrap() and is_challenge_structured() appear to assume that get_current_tan_mechanism() is always present in get_tan_mechanisms(). For this Commerzbank flow, that assumption does not always hold.

*Bank I tested this with
Name of the bank: Commerzbank
FinTS URL:https://fints.commerzbank.de/fints

Expected behavior
After dialog initialization, the client should be able to continue normally with the bank-advertised usable two-step TAN method.

In this case, once the bank reports 900 and 999 as allowed methods, the flow should be able to continue with 900 (photoTAN) without requiring application-level workarounds.

Code required to reproduce

**Describe the bug**

With Commerzbank photoTAN, the client can end up in a state where the current TAN mechanism is still `999`, while the bank-advertised usable two-step mechanism is `900`.

This makes the bootstrap/follow-up flow brittle. In particular, code paths such as `minimal_interactive_cli_bootstrap()` and `is_challenge_structured()` appear to assume that `get_current_tan_mechanism()` is always present in `get_tan_mechanisms()`. For this Commerzbank flow, that assumption does not always hold.

**Bank I tested this with*
Name of the bank: Commerzbank
FinTS URL:https://fints.commerzbank.de/fints

**Expected behavior**

After dialog initialization, the client should be able to continue normally with the bank-advertised usable two-step TAN method.

In this case, once the bank reports `900` and `999` as allowed methods, the flow should be able to continue with `900` (`photoTAN`) without requiring application-level workarounds.

**Code required to reproduce**
```python
import os
from fints.client import FinTS3PinTanClient, NeedTANResponse
from fints.utils import minimal_interactive_cli_bootstrap

client = FinTS3PinTanClient(
    os.environ["FINTS_BLZ"],
    os.environ["FINTS_USER"],
    os.environ["FINTS_PIN"],
    os.environ["FINTS_SERVER"],
    customer_id=os.environ["FINTS_CUSTOMER_ID"],
    product_id=os.environ["FINTS_PRODUCT_ID"],
    product_version=os.environ["FINTS_PRODUCT_VERSION"],
)

client.set_product(
    os.environ["FINTS_PRODUCT_NAME"],
    os.environ["FINTS_PRODUCT_VERSION"],
)

minimal_interactive_cli_bootstrap(client)

with client:
    if isinstance(client.init_tan_response, NeedTANResponse):
        print(client.init_tan_response.challenge)
        tan = input("TAN: ").strip()
        client.send_tan(client.init_tan_response, tan)

    print("current TAN mechanism:", client.get_current_tan_mechanism())
    print("known TAN mechanisms:", list((client.get_tan_mechanisms() or {}).keys()))
    print(client.get_sepa_accounts())

Log output / error message
WARNING:fints.client:Dialog response: 3920 - Zugelassene Ein- und Zwei-Schritt-Verfahren für den Benutzer. (['900', '999'])
TAN bestaetigt.
System-ID:
Bankseitiger Benutzer:
Aktuelle TAN-Methode: 900
Verfuegbare TAN-Methoden:
{'code': '900', 'name': 'photoTAN'}

Additional context
What worked for me as a workaround was:

initialize the dialog normally
allow the client to start with current mechanism 999
after dialog initialization, explicitly switch to the first real two-step method (900)
continue with the photoTAN challenge
After doing that, dialog initialization and SEPA account retrieval worked.