Preserve sibling object keywords in schema previews

[gjermundgaraba]

Summary

Inline schemas with properties beside allOf, anyOf, or oneOf lost those properties, and their required keys, in TypeScript tool previews; only $id-named definitions kept them. Found through a connected MCP server whose create tool requires metadata.type: tools.describe.tool() showed metadata as { [k: string]: unknown }, so agents omitted type and failed the server's runtime validation.

// { type: "object", properties: { type: { type: "string" } }, required: ["type"],
//   allOf: [{ type: "object", additionalProperties: {} }] }
before: { [k: string]: unknown; }
after:  ({ [k: string]: unknown; } & { type: string; })

The vendored compiler is based on upstream json-schema-to-typescript 15.0.4. Upstream fixed the same bug in bcherny/json-schema-to-typescript#708, released in 16.0.0, and added the same intersection re-entry guard in bcherny/json-schema-to-typescript#829, which is still unreleased. This is the minimal equivalent, written before I found the upstream change: named definitions already joined the composition through NAMED_SCHEMA, and inline schemas now take the same path. For the schemas in the new tests, the output matches 16.0.0's, with one exception: this branch names a titled definition after the whole composition, where 16.0.0 still splits it into Metadata and Metadata1. The parser also no longer crashes to unknown when a composed definition's branch refers back to it.

Not in this PR: a pattern-only string allOf still renders as ({ [k: string]: unknown; } & string), and a required key with no matching properties entry is still dropped. Both are separate, pre-existing issues; upstream 16.0.0 renders the first as string.

Verification

• bun run format:check
• bun run lint
• bun run typecheck
• bun run test
• e2e — scenarios/tool-descriptions.test.ts on selfhost; artifact e2e/runs/selfhost/tools-agent-visible-descriptions-snapshot/descriptions.md shows metadata?: ({ [k: string]: unknown; } & { type: string; }).

The Cloudflare and Vercel real-spec snapshots are unchanged, and the Stripe fixture assertions still pass. Known limitation, documented in the vendored README: a null branch of an anyOf/oneOf beside sibling properties is absorbed by the intersection, as named definitions already did and as upstream 16.0.0 renders it.

Checklist

• Added a changeset.
• Added or updated tests for the new behaviour.
• No secrets, credentials, or private data in the diff.