fix(kdf): validate deriveBits output length

[harrshita123]

Fixes #389.

Summary

• validate HKDF and PBKDF2 deriveBits lengths before backend dispatch
• reject values outside the Web IDL unsigned long range
• add shared regression coverage for negative and greater-than-32-bit values

Root cause

The browser backend forwarded arbitrary Dart integers to SubtleCrypto.deriveBits. Web IDL converted those values to unsigned long, so inputs such as -0xfffffff8 wrapped to 8 and produced a one-byte result instead of failing.

The shared validation also prevents oversized PBKDF2 output lengths from reaching the native allocation path.

Validation

• dart analyze --fatal-warnings .
• focused regression on VM
• focused regression on Chrome with dart2js
• focused regression on Chrome with dart2wasm
• broader HKDF/PBKDF2 shared suite on VM
• broader HKDF/PBKDF2 shared suite on Chrome with dart2js
• broader HKDF/PBKDF2 shared suite on Chrome with dart2wasm
• git diff --check