feat(tx-pipeline): implement secure simulate/build/submit pipeline without secret custody

[lishmanTech]

Summary

Implements a secure backend transaction pipeline that supports simulation, transaction building, and submission without ever handling user private keys.

Changes

• Added /simulate endpoint for deterministic validation of transaction intents
• Added /build endpoint to construct unsigned canonical transactions
• Added /submit endpoint for signed transaction submission
• Introduced idempotency key support for safe retry handling
• Ensured backend never stores or requests private keys

Safety

• No secret custody at any stage
• All signing remains client-side

Testing

• Simulate invalid inputs returns consistent error codes
• Duplicate submit requests with same idempotency key return identical results

closes #419

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[Cedarich]

Please fix workflow. Thanks