Add typed input validation for agent creation payload

[Cjay-Cyber-2]

Description — what to implement and the why/impact.

Add runtime validation schemas for agent creation payloads in @astroid/agent to catch configuration errors before requests are dispatched to the Astroid REST API.

Context & Requirements — background, constraints, design references, edge cases;

Background: Developers creating autonomous agents often pass malformed configurations (e.g. invalid Stellar addresses, missing metadata). Validating payloads on the client side improves developer experience and eliminates round-trips for basic errors.
Constraints: Use lightweight validation consistent with existing workspace dependencies or TypeScript type guards.

Acceptance Criteria — a checklist ("- [ ] ...") of specific, testable conditions that define "done".

• Validation function or schema added for agent creation input in @astroid/agent
• Throws descriptive validation errors when required fields or Stellar public keys are invalid
• Full test coverage for valid and invalid agent payloads

Implementation Guidance — likely files/modules to touch (use real paths from the repo context)

Examine packages/agent/src/ and integrate validation prior to API client submission.

Testing & Validation — how the contributor should prove it works

Run pnpm test in packages/agent and verify all positive and negative test cases pass.

Submission Guidelines — must open a PR that includes "Closes #"; assignment is required before starting.

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Wave complexity: Medium