test(conformance): add Anthropic shared-behavior baseline

[moonyue-w]

Summary

• add an offline characterization suite for public behaviors shared with the pinned Anthropic TypeScript SDK baseline
• cover constants, pagination, error inheritance, headers, request IDs, timeouts, and retry semantics
• keep the conformance suite inside the default npm test gate without adding live credentials or network calls

Validation

• npm run build
• npm run test:types
• npm test