Negotiate encrypted payload schema versions

[Dreamstore2046]

Summary

• add explicit encrypted payload version dispatch for enc:v1: and enc:v2: prefixes
• keep enc:v1 export behavior unchanged while allowing importers to accept v2 payloads
• fail closed on unknown encrypted payload versions with MigrationError::IncompatibleVersion
• update encrypted payload docs to describe the versioned encoding-only contract

Closes #1032

Validation

• git diff --check

Notes

• This Windows environment does not have cargo installed, so I could not run cargo test -p data_migration locally. The PR includes focused tests for v1 compatibility, v2 dispatch, and unknown-version rejection for GitHub Actions to run.
• Both enc:v1 and enc:v2 remain base64 marker formats only; they do not add on-chain confidentiality.

[Dreamstore2046]

CI follow-up:\n\n- The Soroban Smart Contracts CI is green on the latest push: Build and Test, Lint (unwrap/expect ban), Security Checks, Storage Key Naming, and Gas Benchmarks all pass.\n- I also carried over the minimal reporting/bill-payments compile unblock that lets the workspace get past the existing Batch-B reporting blockers.\n- Batch-B CI Gate still fails, but it now fails in the unrelated amily_wallet all-features test set: 185 amily_wallet tests fail at FamilyWalletClient::init with HostError: Error(Auth, ExistingValue) / rame is already authorized. The data_migration tests and the new �nc:v1/�nc:v2 negotiation changes are covered by the green Soroban CI.\n\nI am stopping short of broadening this PR into a family_wallet auth-test refactor because that failure is outside the encrypted payload version-negotiation scope for #1032.

[Baskarayelu]

thanks for the contribution! please get assigned to the issue before raising a PR (no assigned issue is linked to you here). closing as invalid for now 🙏