Replace hard-coded contract method strings with a typed enum

[kevindeforth]

Background

Today the MPC node calls NEAR contract methods using string literals, e.g.:

mpc/node/src/indexer/tx_sender.rs

Line 102 in cd9a595

method_name: "get_pending_request".to_string(),

This is error-prone (typos, refactors) and makes it hard to identify the methods our node relies on.

Proposed approach:

• Introduce an enum ContractInterface as the single source of truth for method names.
• Provide as_str() and Display for ergonomic use and logging.
• Add an integration test that probes a contract for a method’s availability (e.g., try a call)
• Replace existing string literals at call sites with ContractMethod::X.as_str().
• Centralize all mappings in one module (e.g., node/src/contracts/methods.rs).

User Story

As an MPC node developer/operator, I want contract method names to be defined in a single, typed place.

Acceptance Criteria

• The contract API required by the node is centralized in a single file.
• all existing string literals for method names of the contract are replaced with enum usage.
• an integration test exists, checking if the methods are available on the contract.

Resources & Additional Notes

No response