Improve overwriting of bytes

[terrier989]

We need to check that the list is modifiable before overwriting.

[Copilot]

Pull request overview

This PR improves the safety of overwriting sensitive byte data by replacing try-catch blocks with proactive runtime type checks. Instead of attempting to overwrite and catching exceptions, the code now uses identical(bytes.runtimeType, Uint8List) to verify the list is modifiable before calling fillRange.

Key Changes:

• Replaced exception-based approach with runtime type checking using identical(runtimeType, Uint8List)
• Simplified code by removing try-catch blocks in favor of conditional checks
• Applied pattern consistently across SensitiveBytes.destroy() and cipher overwriting operations

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
cryptography/lib/src/cryptography/sensitive_bytes.dart	Replaced try-catch with runtime type check in destroy() method to verify bytes are modifiable before overwriting
cryptography/lib/src/cryptography/cipher.dart	Added runtime type checks before overwriting bytes in decryptString() and encryptString() methods to prevent errors on unmodifiable lists

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