Normalize P-521 test fixture for cryptography 47+

[sebasxsala]

Closes #94.

cryptography 47+ rejects tests/keys/ec-p512-private.pem because the P-521 private scalar is encoded as 65 bytes, while P-521 requires a fixed 66-byte private key value.

This updates only the test fixture, preserving the same key material and public key, but normalizing the private key encoding so it can be loaded by newer cryptography versions.

Verified:

• cryptography==47.0.0 loads the normalized fixture
• cryptography==48.0.0 loads the normalized fixture
• uv run pytest passes: 273 passed

[Copilot]

Pull request overview

Updates an EC private-key test fixture so it can be parsed by cryptography 47+ while preserving the same key material/public key, resolving failures when loading the P-521 (ES512) key.

Changes:

• Normalized the P-521 private scalar encoding in tests/keys/ec-p512-private.pem from 65 bytes to the required fixed 66 bytes so cryptography 47+ accepts it.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8817129) to head (b89eadf).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #96   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           47        47           
  Lines         2929      2929           
  Branches       339       339           
=========================================
  Hits          2929      2929           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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