fix(sdk-coin-polyx): route v8 token transfers with dropped sender.did to v8 builder

[nvrakesh06]

Description

Fixes token withdrawals (Polymesh settlement.addAndAffirmWithMediators) failing on the v7/v8 chain migration with:

Invalid transaction: "legs[0].fungible.sender.did" is required

TransactionBuilderFactory.getBuilder() decodes the raw tx against v7 metadata first and only retries against v8 material if that decode throws. v8 wraps leg sender/receiver in an AssetHolder enum ({ portfolio: { did, kind } }) instead of the bare v7 { did, kind }, but the call index for addAndAffirmWithMediators is unchanged between v7/v8 — so decoding a v8-encoded token transfer with v7 metadata does not throw, it just silently produces a decoded transaction missing legs[0].fungible.sender.did. The factory then proceeds with the v7 TokenTransferBuilder, whose schema requires that field directly, and validateDecodedTransaction rejects the transaction during wallet-platform's build-time round-trip re-parse of the withdrawal.

Native transfers (transferWithMemo) are unaffected because that call's v7/v8 arg shape didn't change.

Fix

On the v7 decode-success path for AddAndAffirmWithMediators, detect a missing legs[0].fungible.sender.did and retry via the existing tryGetV8Builder() (which already correctly routes to V8TokenTransferBuilder/V8HexTokenTransferBuilder) before falling through to the v7 builder.

Issue Number

TICKET: CECHO-1471

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• npx tsc --noEmit in modules/sdk-coin-polyx — clean
• npx mocha test/unit/transactionBuilder/*TokenTransfer* — 31 passing
• npx mocha test/unit/transactionBuilder/transactionBuilderFactory.ts — 248 passing, 2 pending
• Root-caused against live staging logs (Loki/wallet-platform) reproducing the exact sender.did failure for a TPOLYX:NVBITGOT withdrawal on testnet Polymesh v8.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My code compiles correctly for both Node and Browser environments
• My commits follow Conventional Commits
• The ticket was included in the commit message as a reference
• I have added tests that prove my fix is effective — TODO before merge: add a regression test to transactionBuilderFactory.ts's suite covering "v7 decode succeeds but sender.did missing → routes to v8 builder"
• New and existing unit tests pass locally with my changes

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