🧹 [Code Health]: Reduce nesting and duplication in test_utils.rs

[ffalcinelli]

🧹 What: Extracted repetitive, deeply nested closures in src/apply/test_utils.rs into two macros: mock_handler! and mock_error_handler!.
💡 Why: The codebase previously duplicated an identical nested structure (capturing an Arc clone, using move ||, and returning an async move block) over 15 times, leading to 6+ levels of indentation and making the mock server setup difficult to read.
✅ Verification: Verified the code using cargo test, cargo clippy, and cargo fmt. All functionality remained identical as the macro exactly mirrors the previous inline implementation.
✨ Result: Substantially improved code maintainability and readability without any functional changes.

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PR created automatically by Jules for task 6419688470836764877 started by @ffalcinelli

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.07%. Comparing base (113c547) to head (00c1d7d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #117   +/-   ##
=======================================
  Coverage   93.07%   93.07%           
=======================================
  Files          33       33           
  Lines        2267     2267           
=======================================
  Hits         2110     2110           
  Misses        157      157           

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