refactor: simplify schema name retrieval in validation messages

[ChihweiLHBird]

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Summary by cubic

Simplified schema name retrieval in OAS validation messages by introducing a single nameOf(Schema) helper. This reduces duplication and ensures consistent messages with a safe fallback.

• Refactors
  • Added private nameOf(Schema) returning name, then title, else "(unnamed)".
  • Replaced inline name/title lookups in null, nullable, and invalid type checks.

^{Written for commit 7d96444. Summary will update on new commits.}

[cubic-dev-ai]

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