Resolve #2271: cover validation contract regressions

[ayden94]

Summary

Add executable regression coverage for documented @fluojs/validation request-boundary contracts.

Linked context: Closes #2271

Changes

• Cover materialize(instance, Target) accepting an existing DTO instance.
• Cover materialize() hydrating fields from DTO binding metadata while preserving validation issue source.
• Cover no-argument @isip() accepting IPv4 and IPv6 and rejecting invalid IP input.

Testing

• pnpm exec biome check packages/validation/src/validation.test.ts
• pnpm --filter @fluojs/validation typecheck
• pnpm --filter @fluojs/validation test

Release impact

• This PR has consumer-visible release impact and includes a changeset.
• This PR has no consumer-visible release impact.

Test-only regression coverage for existing documented behavior; no .changeset/*.md file is required.

Public export documentation

No public exports changed.

• Changed public exports include a source-level summary.
• Changed exported functions document matching @param / @returns tags where applicable.
• Source @example blocks and README scenario examples still play complementary roles.

Behavioral contract

This PR preserves existing documented validation behavior by adding regression tests only.

• No documented behavioral contracts were removed without migration notes.
• New behavioral contracts are documented in the affected package README.
• Intentional limitations are explicitly stated (not silently removed).
• Runtime invariants are covered by regression tests.

Platform consistency governance (SSOT)

No platform contract or governance docs changed.

• SSOT English/Korean mirror structure remains synchronized for changed governance docs.
• If platform contract docs changed, companion updates include discoverability/docs index, tooling or CI enforcement, and regression-test evidence.
• Any package README alignment/conformance claims are backed by createPlatformConformanceHarness(...) tests.