chore: retire unneeded validation adapters

Goal

Retire validation adapters that are no longer needed because their schema libraries support Standard Schema directly.

Keep the Zod adapter supported where its fallback behavior remains required.

Scope

• Confirm which ArkType and Valibot adapter use cases work directly with Standard Schema.
• Identify any supported input/output typing behavior that still requires an adapter.
• Deprecate the ArkType and Valibot packages before removing them from the workspace and publication flow.
• Update examples and documentation to use direct Standard Schema validation where supported.
• Keep Zod adapter guidance and fallback behavior intact.
• Run package type/build tests and affected example validation coverage.

Related Work

This continues the discussion in #3830 where the task should use the current package layout and document the migration path for existing users.

Start by reading the discussion in #3830 and inspecting the current package layout. Check ArkType and Valibot use cases, including input/output typing, against direct Standard Schema support, while preserving Zod fallback guidance. Done means the affected packages are deprecated and removed from workspace/publication flow, examples and documentation show the migration path, and package type/build tests plus affected examples pass.