test(ecdh): enable P-521 tests on Safari

[harrshita123]

Summary

• enable the existing P-521 ECDH vectors when the test suite runs on Safari
• remove the obsolete documentation claiming that Safari does not support P-521
• remove the runtime-detection import that is no longer needed by the ECDH tests

Safari now supports P-521, so excluding these vectors leaves a supported path untested. The existing vectors already cover key import and export across raw, SPKI, PKCS8, and JWK formats, as well as 528-bit key derivation.

Closes #129.

Validation

• dart analyze --fatal-warnings .
• full VM test suite (1,448 passed; 1 platform-specific skip)
• P-521 ECDH tests on Safari 26.5.2 with Dart2JS (41 passed)
• P-521 ECDH tests on Chrome with Dart2JS and Dart2Wasm (82 passed)