🔒 fix: Address loose type comparison vulnerability in Cusip validator

[ronanguilloux]

🎯 What: The validate method in src/IsoCodes/Cusip.php used loose comparison (==) for checking string characters (e.g., '*', '@', '#') and integer checksums.
⚠️ Risk: Loose comparison (==) in PHP can lead to type juggling vulnerabilities (CWE-697). Attackers could potentially craft inputs that evaluate to true due to unexpected type coercion (e.g., "0" equating to 0, or true equating to any non-empty string). This could result in invalid CUSIP values being treated as valid.
🛡️ Solution: Updated all loose comparisons (==) to strict comparisons (===) in src/IsoCodes/Cusip.php. This includes the character checks, the parity check (1 === $i % 2), and the final checksum validation calculation, ensuring types are matched precisely and eliminating the risk of type coercion.

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PR created automatically by Jules for task 14507095163751096891 started by @ronanguilloux

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