fix: Remove empty bank parameter from iDEAL payments

[RRosalia]

Summary

• Fixes iDEAL payment integration to comply with Stripe's updated API requirements
• Removes empty bank parameter that was causing API errors
• Aligns with Stripe's guidance for self-collected iDEAL flows

Changes Made

• Modified confirmIdealPayment in web_stripe.dart to create IdealBankData() without bank parameter
• Added comprehensive tests to verify bank parameter handling
• Updated existing tests to cover the new behavior

Problem Solved

Previously, the plugin was always sending an empty bank parameter even when not provided by developers, causing this Stripe API error:

"You passed an empty string for 'payment_method_data[ideal][bank]'. We assume empty values are an attempt to unset a parameter; however 'payment_method_data[ideal][bank]' cannot be unset. You should remove 'payment_method_data[ideal][bank]' from your request or supply a non-empty value."

References

• Stripe Migration Guide
• Stripe JS confirmIdealPayment docs

Test Plan

• Unit tests pass for all bank parameter scenarios
• Backwards compatibility maintained (existing code won't break)
• Bank parameter is properly omitted from API calls
• No impact on other payment methods

Fixes #2185