testsuite: add argon2 wrong-passphrase test for decrypt_key_file, refs #9686

[mr-raj12]

Description

One test for decrypt_key_file_argon2, refs #9686.

Follow-up to #9686, which added wrong-passphrase coverage for the PBKDF2 path. decrypt_key_file calls decrypt_key_file_argon2 when the stored algorithm is argon2 chacha20-poly1305. Wrong passphrase, different derived key, Poly1305 auth fails, None back. Same contract as PBKDF2.

Added to src/borg/testsuite/crypto/key_test.py:

• wrong passphrase returns None: Argon2 derives a different key, Poly1305 check fails, None returned (not raised)

BORG_TESTONLY_WEAKEN_KDF is set globally in conftest.py, so Argon2 runs with minimal parameters.

No production code changes.

Refs #9686

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests (this PR is the test)
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.95%. Comparing base (7815e02) to head (a6124fc).
⚠️ Report is 6 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9691   +/-   ##
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  Coverage   83.95%   83.95%           
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  Files          93       93           
  Lines       15586    15586           
  Branches     2335     2335           
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+ Hits        13085    13086    +1     
+ Misses       1776     1775    -1     
  Partials      725      725           

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