fix(ffi): reject overflowing cryptographic parameters

[harrshita123]

Fixes #347

Summary

• reject PBKDF2 iteration counts that do not fit Web IDL's unsigned long
• reject RSA-PSS salt lengths that cannot be represented by BoringSSL's signed 32-bit parameter
• add native regression coverage for integer narrowing at the FFI boundary

Root cause

The native backend passed arbitrary-width Dart integers directly to fixed-width FFI parameters. Values outside those native ranges were narrowed before reaching BoringSSL, allowing PBKDF2 iterations and RSA-PSS salt lengths to wrap to different values.

The RSA-PSS path also accepted values such as 0xffffffff, which narrowed to BoringSSL's special -1 salt-length sentinel.

Impact

Oversized parameters now fail before the FFI call instead of silently selecting weaker or otherwise different cryptographic parameters. Valid parameter behavior is unchanged.

Validation

• dart format --output=none --set-exit-if-changed .
• dart analyze --fatal-warnings .
• dart test test/webcrypto_test.dart (1,444 tests passed)