Fix GraphQL private header routing priority mismatch

[Zetazzz]

Summary

Align private GraphQL API routing priority between getSvcKey() and determineMode().

Previously, requests containing both X-Api-Name and X-Schemata could be cached under an api:<db>:<apiName> service key while being resolved through the schemata-header path. That could store an admin-style schema-header API structure under a named API cache key.

This PR makes both paths use the same private header priority:

X-Api-Name > X-Schemata > X-Meta-Schema

Changes

• Added a shared private header mode helper used by both service-key generation and mode selection.
• Kept existing X-Api-Name service-key semantics unchanged.
• Added regression tests for mixed X-Api-Name / X-Schemata requests.

Validation

• pnpm --filter @constructive-io/graphql-server exec jest src/middleware/__tests__/api.test.ts --runInBand
• pnpm --filter @constructive-io/graphql-server build
• pnpm --filter @constructive-io/graphql-server exec jest --runInBand

[Zetazzz]

in short, this's a correctness for a corner case in the private mode:

in getting svckey, the priority was:
if (req.get('X-Api-Name')) return api:...;
if (req.get('X-Schemata')) return schemata:...;
if (req.get('X-Meta-Schema')) return metaschema:...;

X-Api-Name > X-Schemata > X-Meta-Schema

and in getting the value(its mode), the priority was:
if (headers.schemata) return 'schemata-header';
if (headers.apiName) return 'api-name-header';
if (headers.metaSchema) return 'meta-schema-header';

X-Schemata > X-Api-Name > X-Meta-Schema

so there'll be a possible missmatch when there're both 'X-Api-Name' and 'X-Schemata':
the key will be 'api:....'
and the value will be 'schemata-header'

The PR is a small correctness fix with a minor refactor: it centralizes the private header priority so key generation and mode selection cannot diverge again.