Reduce BCrypt cost for faster password hashing in tests

[ashwin47]

Set BCrypt::Engine.cost to MIN_COST (4) in test environment to speed up password hashing operations

Why

BCrypt's default cost factor (12) is designed for production security, where slow hashing protects against brute-force attacks. In tests, we're validating logic—not defending against attackers—so we can safely use the minimum cost without compromising test validity.

Benchmark Results

Branch	Run 1	Run 2	Run 3	Average
main (before)	11.0s	10.0s	9.0s	10.0s
This PR (after)	4.0s	3.0s	4.0s	3.7s

Result: ~63% faster test suite execution

Details

BCrypt Cost Benchmark Comparison
Date: Fri Jan 2 16:30:17 IST 2026

BEFORE (without BCrypt optimization) (main)

→ Run 1: 11.0s - 259 runs, 742 assertions, 0 failures, 0 errors, 0 skips
→ Run 2: 10.0s - 259 runs, 742 assertions, 0 failures, 0 errors, 0 skips
→ Run 3: 9.0s - 259 runs, 742 assertions, 0 failures, 0 errors, 0 skips

AFTER (with BCrypt optimization) (reduce-bcrypt-cost-in-tests)

→ Run 1: 4.0s - 259 runs, 742 assertions, 0 failures, 0 errors, 0 skips
→ Run 2: 3.0s - 259 runs, 742 assertions, 0 failures, 0 errors, 0 skips
→ Run 3: 4.0s - 259 runs, 742 assertions, 0 failures, 0 errors, 0 skips