move vesting update phase into Account.Hardfork.of_stable

[glyh]

This is a request from @Trivo25, having a function exposing the account conversion phase that o1js could refer to, for testing purpose.

Context: https://o1-labs.slack.com/archives/C02BFPS6BA4/p1765170746704759

I also discovered there's a bug when doing this: chunked migration doesn't have vesting param update in it.

[glyh]

!ci-build-me

[glyh]

!ci-build-me

[glyh]

!ci-build-me

[glyh]

!ci-build-me

[glyh]

Rebased onto compatible to resolve merge conflict, applied change request to rename the function

[cjjdespres]

I don't think this necessarily needs a changelog, because the bug it fixes is only active in the ledger sync implementation and that hasn't really been released yet.

[cjjdespres]

!ci-bypass-changelog