fix: txHex needs to be set for the txn in case of TSS

[bhuvanr159]

TICKET: CHALO-349

[linear-code]

CHALO-349

[pritam-gembali]

unit tests?

[pritam-gembali]

The if (unsignedTx?.serializedTxHex) guard at modules/sdk-core/src/bitgo/wallet/wallet.ts:3534-3536 is a silent fallback. If the txRequest comes back in an unexpected shape, the assignment is skipped, no error is raised, and the prebuild is returned with txHex still missing — re-introducing the very bug this PR is fixing, just with an extra HTTP round-trip.

Concrete failure mode

Imagine the backend returns apiVersion: 'full' but transactions is [] (or its first element has no unsignedTx field — e.g. a future API rename):

1. Build returns { txRequestId: 'req-1', stakingParams: {...} } with no txHex.
2. We enter the new branch, fetch the txRequest. ✓
3. txRequest.transactions?.[0]?.unsignedTx → undefined.
4. if (unsignedTx?.serializedTxHex) → false. Block silently skipped.
5. Prebuild is returned without txHex.
6. Caller hits prebuildAndSignTransaction → verifyTransaction at wallet.ts:2189, which sees txPrebuild?.txHex is undefined and either skips verification or throws something downstream that looks unrelated (e.g. a coin-specific verify failing on undefined.length).

The user sees the downstream error and has no breadcrumb pointing back to the real cause seven steps earlier. Debugging means re-running with a debugger.

Suggested fix

if (!unsignedTx?.serializedTxHex) {
  throw new Error(
    `Expected serializedTxHex on TSS resource management prebuild for txRequestId ${prebuild.txRequestId}`
  );
}
prebuild = _.extend({}, prebuild, { txHex: unsignedTx.serializedTxHex });

This matches the assertive style already used at wallet.ts:4193-4202 in prebuildTransactionTxRequests, where missing/malformed txRequest shapes throw explicitly.

Test to update

The new test should leave txHex undefined when txRequest has no serializedTxHex in modules/sdk-core/test/unit/bitgo/wallet/resourceManagement.ts pins the silent-skip behavior as intended. Once that test merges, future reviewers will assume the silent fallback is by design — making it harder to harden later. Please flip it to expect a thrown error:

await wallet.buildResourceDelegations({ delegations: [delegations[0]] })
  .should.be.rejectedWith(/Expected serializedTxHex/);

Happy to approve once the throw is in place and the test is updated.