feat: add tests for key format

[aceppaluni]

Description:

Added unit tests for:

key_format.py.
The goal of this PR is to increase test coverage of key_format.py to be <= 85%

Related issue(s):

Fixes #990

Notes for reviewer:

• Ensured coverage percentage is <= 85%
• Used command pytest --cov=. --cov-report=term-missing tests/unit
• Provided coverage photo

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

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[exploreriii]

Hi @aceppaluni please fix your signing 👍

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[exploreriii]

Request review if available @hiero-ledger/hiero-sdk-python-triage

[nadineloepfe]

great PR! we could also raise a follow up issue to uplift format_key in key_format.py.
First, it would be good to rename the method to format_key.. then it would also be good to check not just for ed25519 but also ECDSA. Currently, when a ECDSA key is passed to format_key, it falls through to the generic fallback.
Would be nice to have a check for ECDSA_secp256k1(hex-encoded).. but that's out of scope for this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main    #1092      +/-   ##
==========================================
+ Coverage   90.91%   91.04%   +0.13%     
==========================================
  Files         139      139              
  Lines        8409     8409              
==========================================
+ Hits         7645     7656      +11     
+ Misses        764      753      -11     

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[exploreriii]

Thanks @aceppaluni !