fix(wasm-mps): migrate legacy EdDSA MPCv2 keyshares

[Marzooqa]

Ticket: WCI-1496

[linear-code]

WCI-1496

[vibhavgo]

Does ed25519_hard_derive_round0_process also need to use decode_keyshare for root_share around line 855?

Since root_share is a persisted EdDSA keyshare, won't wallets DKG'd between June 30 and mid-July still fail deserialization during hardened child derivation (e.g., generating new receive addresses or signing with derived addresses), even though their root-key DSG signing is now fixed?

Could we update line 855 to use decode_keyshare::<EdwardsPoint>(root_share)? and add a test case covering hard derivation with a migrated legacy share?

[Marzooqa]

Does ed25519_hard_derive_round0_process also need to use decode_keyshare for root_share around line 855?

Since root_share is a persisted EdDSA keyshare, won't wallets DKG'd between June 30 and mid-July still fail deserialization during hardened child derivation (e.g., generating new receive addresses or signing with derived addresses), even though their root-key DSG signing is now fixed?

Could we update line 855 to use decode_keyshare::<EdwardsPoint>(root_share)? and add a test case covering hard derivation with a migrated legacy share?

We dont need to do this for hard derivation as thats a feature thats gonna be used for wallet safes and for net new wallets.