fix(en_IN, es_MX, zh_CN): inherit en_GB bank provider so GB iban() is valid

[CedricConday]

These locales fall back to GB IBANs but lacked a correct bank provider, producing invalid IBANs. They now inherit the en_GB bank provider.

Verified against python-stdnum (reference IBAN validator): 300/300 generated IBANs pass stdnum.iban.validate(). Existing tests/providers/test_bank.py suite passes. Includes a regression test.

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AI disclosure: Prepared with AI assistance (Claude / Claude Code). All output was human-reviewed and verified — generated IBANs validated against python-stdnum and the test suite passes.

[fcurella]

these shouldn't inherit from en_GB, but from a more abstract BankIBAN provider. Also, if they don't support IBAn (like China and Mexico), the iban should either raise NotImplemented, or return en_GB as fallback.

[CedricConday]

The three locales in this PR already emit GB IBANs today — the base provider defaults country_code = "GB", so inheriting en_GB makes that explicit rather than changing it.

The same holds for fil_PH, fr_CH, it_CH, ru_RU, tl_PH, and every locale with no bank provider at all: Faker('en_US').iban() returns a GB IBAN too.

Happy to add the abstract IBAN provider and have non-IBAN locales raise, but that breaks iban() for every locale relying on the current default. Would you want that as its own PR against a major version, with this one closed or narrowed to just the type annotation?