fix: handle specifiedByURL field on __Type introspection

[shkuls]

Problem

specifiedByURL is advertised as a field on __Type in the introspection schema, but querying it returns an error:

{"data": null, "errors": [{"message": "unexpected field specifiedByURL type on __Type"}]}

This breaks GraphQL clients that do 2-step introspection — they first ask what fields __Type supports, see specifiedByURL listed, then include it in their full introspection query and crash. Affected tools include Apollo Kotlin, graphql-codegen, and Rover CLI.

Reported in:

• GraphQL integration: specifiedByURL is surfaced in introspection but is 'unexpected' when selected  supabase#31158 (Dec 2024)
• specifiedByURL field on __Type is declared in the schema but not handled by the resolver #629 (Apr 2026)

Root cause

Three systems were out of sync:

1. src/graphql.rs — __TypeType::fields() advertised specifiedByURL ✓
2. src/builder.rs — __TypeField enum had no SpecifiedByURL variant, and the query parser's match fell through to the _ => Err(...) catch-all ✗
3. src/transpile.rs — __TypeBuilder::serialize() had no arm to write the field ✗

Fix

Three small additive changes:

• Add specified_by_url() -> Option<String> to the ___Type trait with a default of None
• Add SpecifiedByURL variant to the __TypeField enum
• Add "specifiedByURL" => __TypeField::SpecifiedByURL match arm in the query builder
• Add serializer arm that calls self.type_.specified_by_url() (returns null for all current types)

No existing behaviour changes. All types correctly return null since pg_graphql has no custom scalars with a specification URL.

Test

Added test/sql/resolve_specified_by_url.sql covering:

• specifiedByURL on a built-in scalar (Boolean) returns null
• specifiedByURL alongside other fields (String) returns null
• specifiedByURL on a non-scalar type (Query) returns null

Repro (before fix)

# Step 1: specifiedByURL is advertised
curl -s -X POST http://localhost:54321/graphql/v1 \
  -H "Content-Type: application/json" \
  -d '{"query":"{ __schema { types { name fields { name } } } }"}' \
  | grep specifiedByURL   # → found

# Step 2: querying it crashes
curl -s -X POST http://localhost:54321/graphql/v1 \
  -H "Content-Type: application/json" \
  -d '{"query":"{ __type(name: \"Boolean\") { specifiedByURL } }"}'
# → {"data": null, "errors": [{"message": "unexpected field specifiedByURL type on __Type"}]}