feat(sdk-core): add automatic signature cleanup for Express TSS signing

[dpkjnr]

Fixes Express API gap where re-signing partially signed Full TxRequests would fail. The /signtxtss route now automatically detects and cleans up partial signature shares before attempting to sign.

Changes:

• Add ensureCleanSigSharesAndSignTransaction() to Wallet class
• Update Express handleV2SignTSSWalletTx() to use new method
• Add comprehensive unit tests for signature cleanup logic
• Check signature shares presence instead of transaction state

Benefits:

• Fixes re-signing failures for partially signed transactions
• Backward compatible with existing signing flows
• No API changes required
• Keeps tssUtils properly encapsulated

Ticket: COIN-5989

[nvjsr]

@claude review

[Copilot]

Pull Request Overview

This PR fixes an API gap in the Express TSS signing endpoint where re-signing partially signed Full TxRequests would fail due to stale signature shares. The solution adds automatic signature cleanup before signing operations.

• Adds signature share cleanup logic to detect and clean partial signatures before signing
• Updates Express TSS signing endpoint to use the new cleanup method
• Provides comprehensive test coverage for the signature cleanup scenarios

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
modules/sdk-core/src/bitgo/wallet/wallet.ts	Adds ensureCleanSigSharesAndSignTransaction method with signature cleanup logic
modules/express/src/clientRoutes.ts	Updates Express TSS signing to use new cleanup method
modules/bitgo/test/v2/unit/wallet.ts	Adds comprehensive unit tests for signature cleanup functionality

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