fix(sdk-core): decode accounts without created timestamps

[joshglogau]

Summary

• Allow API.v1.AccountObject.created to be omitted and expose it as TimePoint | null.
• Preserve typed get_account decoding for valid system-account responses with reduced metadata.
• Document the compatibility behavior and add a focused regression fixture.

Root cause

The Sandbox sysio account is valid, but its get_account response omits the legacy created field. The decoder treated that field as required and rejected the entire response.

Impact

SDK consumers can query these system accounts normally without falling back to untyped responses. Consumers must handle created === null when an RPC omits the timestamp.

Validation

• pnpm --filter @wireio/sdk-core compile
• pnpm --filter @wireio/sdk-core typecheck:strict-null
• Focused ESLint checks
• Chain.test.ts: 14 tests passed
• Live Sandbox verification decoded sysio with created: null