feat(bitgo): add support for Export txn type in prebuildTransaction

[mohd-kashif]

Summary

• Add Export, Import, and ImportToC cases to the TSS prebuildTransactionTxRequests switch for MPC cross-chain transfers
• Add export case to the EVM populateIntent switch in mpcUtils.ts
• Add unit tests for all new cases

Context

MPC (TSS) wallets use prebuildTransactionTxRequests which routes transaction types to the /txrequests endpoint via prebuildTxWithIntent. The existing multisig flow bypasses this switch entirely — it POSTs
directly to /tx/build where params.type is forwarded as-is. That's why multisig cross-chain works but MPC wallets hit "transaction type not supported: Export".

This PR adds the three Avalanche/Flare atomic transaction types to the TSS prebuild switch:

params.type (from caller)	intentType (sent to WP)	Direction
Export	export	C→P or P→C export
Import	import	Import into P-chain
ImportToC	importtoc	Import into C-chain

The Export case passes recipients, nonce, feeOptions, and feeToken since C-chain exports need gas pricing. Import and ImportToC only pass recipients since P-chain fees are computed server-side.

The populateIntent EVM switch also needs the export case because FLR is EVM (baseCoin.isEVM() === true), so without it the intent would throw "Unsupported intent type export" at the EVM default case.
Import and ImportToC don't need EVM cases because they originate from FLRP which is non-EVM and uses the generic fallback path.

Changes

File	Change
modules/sdk-core/src/bitgo/wallet/wallet.ts	Add case 'Export', case 'Import', case 'ImportToC' to prebuildTransactionTxRequests switch
modules/sdk-core/src/bitgo/utils/mpcUtils.ts	Add case 'export' to EVM populateIntent switch
modules/bitgo/test/v2/unit/wallet.ts	Add 4 unit tests: 3 for prebuild cases + 1 for populateIntent

Test plan

• yarn unit-test --scope bitgo -- --grep "Export" — 3 prebuild tests pass
• yarn unit-test --scope bitgo -- --grep "export intent" — populateIntent test passes
• Existing wallet tests still pass (no regression)

[linear-code]

CECHO-1029