fix(hmac): use block size for default key length

[harrshita123]

Summary

• use each hash function's block size as the default native HMAC key length
• remove stale backend adapter defaults so omitted lengths remain consistent
• document the default lengths and add regression coverage for every supported hash

Root cause

The FFI backend used EVP_MD_size when length was omitted. That function returns the digest size, while Web Crypto defines the default HMAC key length as the hash function's block size. This made native-generated keys shorter than browser-generated keys.

Explicitly supplied key lengths continue to take precedence.

Validation

• dart format --output none --set-exit-if-changed .
• dart analyze --fatal-warnings .
• dart test test/hmac_default_key_length_test.dart -p vm
• dart test test/hmac_default_key_length_test.dart -p chrome -c dart2js,dart2wasm
• dart test --exclude-tags browser-interop -p vm

Closes #342