fix(lang): Enforce mut constraint on Migration accounts

[spellsaif]

Migration<'info, From, To> relies on AccountsExit::exit to persist the migrated state to storage. Because exit.rs skips generating exit routines for non-mutable fields (match f.constraints.is_mutable() { false => quote! {} }), declaring a Migration without #[account(mut)] resulted in silent state loss despite the instruction succeeding.

This PR adds a compile-time check in constraints_cross_checks requiring Migration accounts to be marked mut.

Testing

• Added unit tests in lang/syn/tests/migration_mut.rs verifying:
  • Migration without #[account(mut)] produces a compile-time parse error.
  • Migration with #[account(mut)] parses successfully.
• cargo test -p anchor-syn passed.
• cargo clippy -p anchor-syn -- -D warnings passed (0 warnings).

[vercel]

@spellsaif is attempting to deploy a commit to the OtterSec Team on Vercel.

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[0x4ka5h]

fix CI

[spellsaif]

@0x4ka5h can you rerun it again it. I think it is rate limit problem

[swaroop-osec]

lgtm