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
• keep omitted lengths consistent across backend adapters
• document the Web Crypto default and distinguish block size from digest size
• run default-length regression coverage through the shared native and browser test suite

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 .
• focused shared regression on the VM
• focused shared regression on Chrome with dart2js and dart2wasm
• full VM suite: 1,462 passed, 1 platform-specific skip

Closes #342