fix(ext/node): implement ECDH.convertKey and fix related ECDH bugs (#32532)
## Summary
- Implement `ECDH.convertKey()` static method supporting compressed,
uncompressed, and hybrid point formats
- Fix `setPrivateKey()` not decoding string keys with encoding parameter
- Fix `getPublicKey()` not supporting hybrid format
- Fix `op_node_ecdh_compute_public_key` using `to_sec1_bytes()`
(compressed, 33 bytes) instead of `to_encoded_point(false)`
(uncompressed, 65 bytes), which caused a panic on `copy_from_slice`
- Enable `test-crypto-ecdh-convert-key.js` in node_compat config
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>