This PR fixes value coercion during subgraph schema expansion. Previously, directive argument values and field argument defaults were not being properly coerced after directive definitions were expanded, leading to validation failures for schemas that relied on type coercion rules.

When a subgraph schema uses shorthand syntax like @key(fields: ["id"]) (a single-element list where a String is expected), the schema would fail validation because the coercion from ["id"] → "id" wasn't happening during the expansion phase.

• Added coerce_schema_values() which runs after directive definitions are expanded in expand_schema(). This function coerces:
  • Directive argument values on object types and fields
  • Field argument default values on object and interface types
  • Input field default values on input object types
  • Directive definition argument defaults

Checklist

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• PR description explains the motivation for the change and relevant context for reviewing
• PR description links appropriate GitHub/Jira tickets (creating when necessary)
• Performance impact assessed and acceptable
• Tests added and passing¹
  • Manual tests, as necessary

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