Split the proto_ingestion schema spec into cohesive files

[jwils]

Why

elasticgraph-proto_ingestion's schema_spec.rb still held 34 examples covering every proto schema generation concern, and "schema" is a broad enough label that it would keep growing. Per review feedback on #1306, the remaining scalar examples belong in scalar_proto_types_spec.rb, and the other cohesive groups deserve their own files too.

What

Move each cohesive group of examples out of schema_spec.rb:

File	Covers	Examples
scalar_proto_types_spec.rb (added by #1306)	how scalar types map to proto field types, including import: and field_comment:	9 existing + 4 moved here
stable_numbers_spec.rb	field and enum value numbers staying stable as the schema evolves	8
external_proto_enums_spec.rb	external_proto_enum, both sourcing values and referencing an external enum type	10
abstract_types_spec.rb	oneof wrappers for interface and union types	3
proto_syntax_and_headers_spec.rb	the syntax: and header_lines: options	3

schema_spec.rb keeps the 6 examples that cover schema generation as a whole, dropping from 1064 lines to 172.

Every example moved verbatim — I verified that all 43 example bodies are byte-identical before and after the split, and the suite example count is unchanged at 107. schema_edge_cases_spec.rb is untouched.

Risk Assessment

None — spec file organization only. No behavior change, no coverage change (elasticgraph-proto_ingestion stays at 100% line and branch coverage).

References

• Addresses Map DateTime to google.protobuf.Timestamp and document temporal string formats #1306 (comment)
• Stacked on Use Buf to guard protobuf schema compatibility #1352 (→ Source enum values from external proto enums and reference external enum types #1286)