feat: add fuzzing infrastructure and tests

[a1denvalu3]

Summary

Introduces property-based fuzz testing using hypothesis to improve the robustness of core data models and the Mint API. This ensures data integrity during serialization and verifies that the API handles unexpected inputs gracefully without crashing.

Changes

• Core Fuzzing (tests/fuzz/test_fuzz_core.py):

  • Implements round-trip serialization/deserialization tests for Secret, BlindedMessage, Proof, MintQuote, MeltQuote, and Keyset models.
  • Verifies model invariants and state consistency (e.g., a quote cannot be both 'paid' and 'pending').
  • Tests TokenV3 to TokenV4 conversion integrity.

• API Fuzzing (tests/fuzz/test_fuzz_mint_api.py):

  • Adds fuzz tests for critical Mint endpoints: /mint, /melt, /swap, /checkstate, /restore.
  • Validates that the server handles malformed, random, or boundary-case inputs gracefully without crashing (500 Internal Server Error).
  • Asserts that response status codes are within expected handled ranges (200, 400, 404, 422, 503).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.52%. Comparing base (516cee2) to head (ee12d6a).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #864      +/-   ##
==========================================
- Coverage   64.71%   61.52%   -3.20%     
==========================================
  Files          91       90       -1     
  Lines       10900    10838      -62     
==========================================
- Hits         7054     6668     -386     
- Misses       3846     4170     +324     

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