feat(sdk-coin-flpr): implement MPC support and enhance transaction signing methods

[mohd-kashif]

Summary

• Add TSS (MPC ECDSA) support to sdk-coin-flrp, making FLR the first P-chain coin in BitGoJS to support TSS
• Enable threshold=1 signing for MPC wallets alongside existing threshold=2 multisig
• Add addExternalSignature() for injecting MPC-produced signatures into transactions
• Add getSignablePayload() and addSignatureToTransaction() for the MPC signing flow
• Add MPC single-address path in isWalletAddress() validation
• Add comprehensive MPC e2e tests for all 4 cross-chain builders (ExportInC, ExportInP, ImportInC, ImportInP)

Changes

src/flrp.ts

• Add supportsTss() returning true and getMPCAlgorithm() returning 'ecdsa'
• Add getSignablePayload(txHex) — returns raw bytes for MPC ceremony to sign
• Add addSignatureToTransaction(txHex, signature) — injects MPC signature into unsigned tx
• Update isWalletAddress() to handle MPC wallets (single address, single keychain)

src/lib/transaction.ts

• Add addExternalSignature(signature) — fills empty credential slots with an externally-produced 65-byte ECDSA signature, works for both C-chain and P-chain transactions

src/lib/transactionBuilder.ts

• Relax validateThreshold() to accept 1 (MPC) or 2 (multisig)

src/lib/atomicTransactionBuilder.ts

• Update getSigningAddresses() to return 1 address for threshold=1 (MPC)

Tests (18 new MPC tests + 7 threshold/TSS tests = 25 total)

• Each builder has a MPC signing (threshold=1) suite testing: build+sign(), build+addExternalSignature(), sign()==addExternalSignature() equivalence, deserialize+sign roundtrip
• ExportInP additionally tests change output threshold=1 and single sigIndex
• Threshold validation tests (accept 1/2, reject 0/3)
• TSS declaration tests (supportsTss, getMPCAlgorithm, MPC wallet address)

Test plan

• npx tsc --noEmit — TypeScript compiles clean
• All 335 unit tests passing (was 317, +18 new MPC builder tests)
• All 25 MPC-specific tests pass (--grep "MPC")
• Existing multisig tests unaffected — threshold=2 flow unchanged
• Verify MPC signing produces valid transactions on Coston2 testnet (follow-up)

[linear]

CECHO-674 [BitGoJS] Make sdk-coin-flrp TSS-aware