fix(search-api-graphql): tell the caller which argument they got wrong

[ddeboer]

Fix #730

Stacked on #728 (the IRI scalar this touches lives there); base retargets to main once that merges.

An out-of-range perPage (or page) was a plain Error thrown from a resolver, which graphql-yoga masks as "Unexpected error.". The sentence saying what was wrong survived only in the API container’s log – unreadable to a presentation-layer developer building against a hosted endpoint.

Changes

• build-schema.ts – a userError() helper throwing a GraphQLError with extensions.code = 'BAD_USER_INPUT', used for the paging bounds and the IRI scalar’s coercion failure. The message now reaches the client, and the code lets a consumer tell “fix your query” from “retry later” without matching on prose. "Unexpected error." is left for faults nobody anticipated – an unreachable engine still masks as before.
• SDL descriptions on every root-field argument, including the perPage bound (Results per page, between 0 and 100. …), so the playground’s own documentation answers where the boundary is before a request has to fail to say it. Complements feat(search-api-graphql): write the GraphQL contract of a schema module to a file #722.
• Docs – a new “Errors the caller can fix” section in docs/reference/search-api-graphql.md, plus a note that both paging bounds are in the SDL.
• Tests – a handler test asserting the message and code survive the transport (the direct-execution tests bypass masking, which is exactly where the bug lived), a counterpart asserting an engine failure still masks, and a code assertion on the existing paging-bounds test.

The generated SDL changes shape: arguments now carry descriptions, so printSchema breaks the root field over several lines. Snapshot updated; a consumer with its own contract snapshot will see the same diff.