feat: add banks list for en_GB locale

[osolomientsev]

What does this change

Adds a banks tuple to the en_GB bank provider, enabling the bank() method to return realistic UK bank names. Also adds a test_bank test to the TestEnGb class in tests/providers/test_bank.py.

What was wrong

The en_GB bank provider was missing the banks attribute, causing bank() to raise AttributeError despite being documented as supported for this locale.

How this fixes it

Added a banks tuple containing major UK retail banks, challenger banks, and Islamic banks. Sources:

• https://en.wikipedia.org/wiki/List_of_banks_in_the_United_Kingdom
• https://en.wikipedia.org/wiki/Banking_in_the_United_Kingdom

Fixes #2280

AI Assistance Disclosure (REQUIRED)

• If AI tools were used, I have disclosed which ones, and fully reviewed and verified their output.

Claude (Anthropic) was used to assist with compiling the initial bank list. The list was cross-referenced against Wikipedia sources and verified manually.

Checklist

• I have read the documentation about CONTRIBUTING
• I have read the documentation about Coding style
• I have run make lint

[reory]

"Hi @osolomientsev!
I previously worked on this exact issue (PR #2332) and it's great to see it being pushed forward with tests and a more comprehensive list including challenger banks. The logic looks solid—happy to see en_GB finally getting its bank provider!" 😁

[fcurella]

Thank you!