Trace source: document spanLinksValueExpression in the v2 sources external API (OpenAPI parity) #2695

Summary

#2463 added a Span Links Expression field to the trace source form and schema (spanLinksValueExpression), mirroring Span Events Expression. The field already round-trips through the v2 sources API at runtime because it lives on the shared source schema, but the OpenAPI JSDoc annotation in packages/api/src/routers/external-api/v2/sources.ts is missing. Its sibling spanEventsValueExpression is documented there, so this is a parity gap in the API docs only.

Follow-up

• Add the spanLinksValueExpression property to the trace source block in packages/api/src/routers/external-api/v2/sources.ts (mirror spanEventsValueExpression).
• Regenerate openapi.json (yarn workspace @hyperdx/api run docgen) and confirm lint:openapi passes.

No runtime behavior change. Split out of #2463 to keep that PR app-only and focused.