Introduce runtime validation for public request inputs on critical mutations

[David-patrick-chuks]

Summary

Add runtime validation for high-risk request inputs such as payment execution, wallet provisioning, and agent creation.

Why this matters

TypeScript cannot protect every runtime caller. Validation improves safety for JavaScript consumers and helps catch malformed requests before they become harder-to-debug backend errors.

Scope

• Identify critical mutation methods.
• Add validation strategy for required fields and value shapes.
• Ensure validation errors map cleanly into SDK error types.
• Keep validation implementation maintainable and contributor-friendly.

Acceptance Criteria

• Critical mutation methods reject invalid inputs early.
• Validation errors are typed and documented.
• Tests cover invalid input scenarios.
• Validation approach does not make the SDK difficult to evolve.

Dependencies

• Depends on Align core domain models with Lily backend response envelopes #1