PTHMINT-96: Webhook validation for payloads with Unicode characters

[zulquer]

This pull request improves the handling of unicode characters in webhook validation and enhances the test coverage to ensure correct behavior with non-ASCII payloads. The main changes are grouped into improvements in unicode support and expanded test coverage.

Unicode handling improvements:

• Updated the json.dumps call in src/multisafepay/util/webhook.py to use ensure_ascii=False, allowing unicode characters to be preserved in the compact JSON used for HMAC signature calculation.

Test coverage enhancements:

• Added a new test, test_validate_with_unicode_characters, to verify that webhook validation works correctly when the payload contains unicode characters. This test ensures the signature is generated and validated as expected with non-ASCII data.
• Imported additional modules (base64, hashlib, hmac, time) in tests/multisafepay/unit/util/test_unit_webhook.py to support the new unicode validation test.- Update Webhook.validate to use ensure_ascii=False during JSON reconstruction.
• Add unit test for Unicode support in webhook validation (e.g., 'ñ', '€').

[Copilot]

Pull request overview

This PR fixes webhook validation to correctly handle Unicode characters in payloads by ensuring the HMAC signature calculation preserves non-ASCII characters instead of escaping them.

Key changes:

• Modified JSON serialization in webhook validation to preserve Unicode characters
• Added comprehensive test coverage for Unicode character handling in webhook payloads

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/multisafepay/util/webhook.py	Added ensure_ascii=False parameter to json.dumps to preserve Unicode characters during signature validation
tests/multisafepay/unit/util/test_unit_webhook.py	Added new test case with Unicode characters and imported necessary cryptographic modules

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.69%. Comparing base (41f42f2) to head (323255c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #42   +/-   ##
=======================================
  Coverage   90.69%   90.69%           
=======================================
  Files         143      143           
  Lines        2471     2471           
=======================================
  Hits         2241     2241           
  Misses        230      230           

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