🧪 [testing improvement] Add test for set_target_realm

[ffalcinelli]

🎯 What: Added a unit test test_set_target_realm to cover the previously untested set_target_realm setter method in src/client.rs.
📊 Coverage: The test ensures that instantiating a client creates it with an empty realm, and that set_target_realm successfully mutates the state to a new provided realm value.
✨ Result: Test coverage for the KeycloakClient logic in src/client.rs is improved, providing an explicit safeguard against regression.

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PR created automatically by Jules for task 14726885615570118195 started by @ffalcinelli

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.07%. Comparing base (038df20) to head (3a59790).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #134   +/-   ##
=======================================
  Coverage   92.07%   92.07%           
=======================================
  Files          27       27           
  Lines        2208     2208           
=======================================
  Hits         2033     2033           
  Misses        175      175           

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