fix(upstream): ReadChallengeTx rejects spec-compliant client_domain

[0dillon]

Summary

The official Go SDK's ReadChallengeTx rejects spec-compliant client_domain challenges because it expects every operation after the first to be sourced at the server account. SEP-10 requires the client_domain manage data operation to be sourced at the client domain's key. We should submit a PR upstream to fix this.

Acceptance Criteria

• Fork stellar/go-stellar-sdk and fix ReadChallengeTx.
• Submit PR to upstream repository.
• Once merged, update Anchorage to use the fixed SDK and retire our custom reader if possible.

Tech Stack

Go, Stellar SDK