feat(express): add API endpoint to verify private key matches wallet

[bitgo-ai-agent-dev]

What

• Add POST /api/v2/{coin}/wallet/{id}/verifyPrivateKey endpoint to BitGo Express
• The endpoint accepts either a plaintext private key (prv) or an encrypted private key (encryptedPrv + walletPassphrase)
• Fetches the wallet's user keychain from BitGo automatically when publicKey is not provided (uses commonKeychain for TSS wallets, pub for standard wallets)
• Delegates cryptographic verification to coin.assertIsValidKey, which handles both standard (onchain) and TSS wallet key types
• Returns { valid: true } on success; returns 400 for missing required params; propagates errors for key mismatches

Why

• Users need a way to verify that a private key they hold actually corresponds to a specific wallet's user keychain, without having to sign a transaction
• This is useful for key auditing, recovery workflows, and onboarding validation where operators want to confirm they have the correct key before committing to a transaction

Test plan

• 24 unit tests passing covering codec validation, route definition, successful verification, automatic public key fetching, TSS key type, encryptedPrv flow, and all error cases (missing prv, missing walletPassphrase, key mismatch, wallet not found)
• Manually test with a real BTC wallet using POST /api/v2/tbtc/wallet/:id/verifyPrivateKey with valid and invalid keys

Ticket: WCI-1139