🧹 Refactor TokenResponse to local scope

[ffalcinelli]

🎯 What: Moved the TokenResponse struct definition from the module scope in src/client.rs into the login function.
💡 Why: TokenResponse is a single-use Data Transfer Object (DTO) only needed when parsing the JSON response of the login token endpoint. Defining it globally unnecessarily pollutes the module scope. Scoping it locally inside the function improves maintainability by explicitly isolating its lifecycle and keeping related code grouped together.
✅ Verification: Verified the fix by running the full test suite (cargo test), ensuring all tests pass (ignoring the unrelated known failure of test_real_keycloak_integration due to docker overlayfs).
✨ Result: Improved module cleanliness and code encapsulation without altering existing behavior.

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PR created automatically by Jules for task 12498791933533764945 started by @ffalcinelli

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.86%. Comparing base (db6a2d7) to head (06e7a25).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
- Coverage   92.94%   92.86%   -0.09%     
==========================================
  Files          33       33              
  Lines        2239     2241       +2     
==========================================
  Hits         2081     2081              
- Misses        158      160       +2     

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