Harden OpenAPI ingest for large real-world specifications

[SonAIengine]

Summary

• resolve OpenAPI references lazily per path instead of expanding and renormalizing the entire document
• handle literal schema properties named $ref, circular references, and escaped JSON Pointer segments safely
• cache schema leaf extraction within one ingest to avoid repeated traversal of shared response contracts
• add regression coverage for lazy root resolution, shared caches, circular schemas, and Kubernetes-style $ref properties

Real-world validation

• Stripe official OpenAPI (4.48 MB): 615 tools in 21.89s, max RSS about 1.05 GB; baseline timed out at 120s after exceeding 5.18 GB
• Kubernetes official Swagger (4.48 MB): 1,202 tools in 4.75s, max RSS about 271 MB; baseline failed on a literal $ref schema property

Verification

• poetry run ruff check .
• poetry run ruff format --check .
• poetry run pytest tests/ -q (1240 passed, 47 skipped)
• poetry build