fix(sdk-coin-polyx): forward decoded specVersion to v8 transfer builders in tryGetV8Builder

[nvrakesh06]

Problem

tryGetV8Builder() in TransactionBuilderFactory hardcodes utils.getV8Material() (static specVersion: 8000000) on all returned v8 builders. The live Polymesh chain is at specVersion: 8000020. createBaseTxInfo() reads specVersion from this._material and encodes it in the signing payload's additional bytes — so any builder returned by tryGetV8Builder without a material override produces a rebuilt payload with wrong additional bytes, causing verifySignature to fail even though the TSS signature is cryptographically valid.

Root Cause

tryGetV8Builder decoded the transaction against v8 metadata correctly, but then created builders without forwarding the decoded specVersion — always defaulting to the static placeholder. The analogous fix for getV8BatchStakingBuilder was done in SI-1034 on the decode-success path in getBuilder(), but the four v8 transfer builders in tryGetV8Builder were missed.

Fix

Build v8MaterialWithDecodedSpec from the decoded transaction bytes:

const v8MaterialWithDecodedSpec = {
  ...v8Material,
  specVersion: decodedTxn.specVersion ?? v8Material.specVersion,
  txVersion: decodedTxn.transactionVersion ?? v8Material.txVersion,
} as Interface.Material;

• decodedTxn.specVersion is populated for signing payloads (unsigned txs) — the production path used by getSignedTx/verifySignature
• Falls back to v8Material.specVersion for signed extrinsics where decodeSignedTx doesn't return specVersion, so validateBaseFields doesn't throw

All four v8 transfer builders now receive .material(v8MaterialWithDecodedSpec):

• V8TransferBuilder
• V8HexTransferBuilder
• V8TokenTransferBuilder
• V8HexTokenTransferBuilder

Relationship to wallet-platform fix

A defensive wallet-platform fix (re-applying .material(material) after factory.from() in getSignedTx) is tracked in bitgo-microservices PR #61275. Both fixes arrive at the correct specVersion independently — having both is defense-in-depth with no conflict.

Tests

Added regression test in transactionBuilderFactory.ts that:

1. Builds an unsigned v8 token transfer with live specVersion: 8000020
2. Decodes it via TransactionBuilderFactory.from() with the same live material
3. Asserts signablePayload byte-matches the original

All 252 existing unit tests pass.

References: SI-1034

[linear-code]

SI-1034