fix(sdk-coin-polyx): stub getMaterial in recovery unit tests

[yashvanthbl137-crypto]

Summary

• Stub getMaterial in POLYX recovery unit tests so transaction build and decode use the same bundled metadata
• Fixes CI failures in @bitgo/sdk-coin-polyx after Polymesh testnet upgraded to runtime v8 (specVersion: 8000000)
• Matches the existing pattern in @bitgo/sdk-coin-dot recovery tests

Problem

Recover Transactions unit tests were failing:

• should recover a tx for non-bitgo recoveries
• should recover a txn for unsigned-sweep recoveries
recover() called live getMaterial() (v8 metadata from wss://testnet-rpc.polymesh.live) while getBuilder().from(serializedTx) decoded using bundled v7 metadata (specVersion: 7002000). The mismatch caused decode errors (e.g. treasury.disbursement, system.killStorage).

Fix

Test-only change in modules/sdk-coin-polyx/test/unit/polyx.ts:

getMaterialCB = sandBox.stub(Polyx.prototype, 'getMaterial' as keyof Polyx);
getMaterialCB.resolves(utils.getMaterial(coins.get('tpolyx').network.type));
Production recover() is unchanged and still fetches live metadata for real broadcasts.

Ticket : CGD-1862