fix(payment-methods): rename gddy payments to gddy payment-methods (DEVEX-900)

[sswaminathan-godaddy]

Summary

• Rename the gddy payments command group to gddy payment-methods (module rust/src/payments/ -> rust/src/payment_methods/, clap group name, telemetry with_system tag).
• Update cross-referencing help/error text in domain purchase, domain/common.rs 402 error message, and README.md.
• Rename the common.rs test to match (payment_required_error_points_to_payment_methods_add).

rust/schemas/api/payments.json and the API-explorer "payments" API domain catalog entry are untouched (unrelated Payments API domain).

Test plan

• cargo build
• cargo test (253 passed)
• cargo clippy -- -D warnings (clean)
• cargo run -- payment-methods add --help shows correct help text
• cargo run -- payments add errors with "unknown command" (old name removed)
• cargo run -- domain purchase --help cross-reference text says gddy payment-methods add

DEVEX-900