Validate protobuf enum prefix collisions

[jwils]

Why

Distinct GraphQL enum names can normalize to the same protobuf enum-value prefix, producing duplicate package-level symbols in schema.proto.

What

• Validate enum-value prefixes across the enum types actually emitted in schema.proto.
• Raise an actionable schema error naming both enum types and their shared prefix.
• Preserve reachability behavior by allowing collisions with enum types that are not emitted.

Validation

• script/run_gem_specs elasticgraph-proto_ingestion
• script/lint elasticgraph-proto_ingestion
• script/type_check

Risk assessment

Low. This adds validation only for schemas that would otherwise emit invalid protobuf definitions.

References

• Generate a protobuf schema artifact from ElasticGraph schemas #1080 (comment)
• Follow-up to Generate a protobuf schema artifact from ElasticGraph schemas #1080