Add OpenAPI docs generation

[KayleeWilliams]

Summary

• Add native OpenAPI YAML/JSON to MDX API reference generation for leadtype docs builds.
• Wire generated API pages into nav, search, llms/package docs, markdown flattening, and exported API component prop types.
• Dogfood the feature in the Fumadocs example with a generated Leadtype REST API page and renderer components.
• Document the authoring flow and add a changeset for the leadtype package.

Checks

• bun run --filter leadtype check-types
• bun run --filter leadtype lint
• bun run --filter leadtype test -- src/openapi/openapi.test.ts src/internal/package-surface.test.ts
• bun run --filter leadtype build
• bun run --filter fumadocs-example check-types

Note: the full leadtype test hook still sees unrelated untracked docs under docs/authoring with existing missing-link lint failures, so the commit was created after the scoped checks above passed.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/openapi-docs-generation

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