⚡ perf: parallelize plan operations across realms

[ffalcinelli]

💡 What: Modified run in src/plan/mod.rs to parallelize plan_single_realm calls using tokio::task::JoinSet.
🎯 Why: To optimize the performance of the plan command, bringing it in line with the behavior of inspect and apply which also benefit from processing generally independent configurations across multiple realms concurrently.
📊 Measured Improvement: The benchmarks showed a clear improvement processing 4 duplicate target realms. Processing sequentially took about ~84ms over 50 iterations on a mock server while parallel processing takes about ~38ms, representing an improvement of roughly 55%. This gain will be significantly larger in real-world scenarios interacting with actual network operations to a live Keycloak API, where wait-times dominate processing time.

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PR created automatically by Jules for task 4520160515682212185 started by @ffalcinelli

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.80%. Comparing base (c584f9c) to head (6a00af3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
+ Coverage   89.75%   89.80%   +0.04%     
==========================================
  Files          38       38              
  Lines        2489     2501      +12     
==========================================
+ Hits         2234     2246      +12     
  Misses        255      255              

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