Add OpenAI aggregation gateway

[guweigang]

Summary

• add OpenAI aggregation gateway configuration, routing, plugin planning, and HTTP dispatch under /v1
• add OpenAI-compatible chat/responses runtime with passthrough streaming, mapped NDJSON/SSE output, fallback, executor backends, and response registry support
• add vjsx plugin call/runtime support plus OpenAI gateway examples for OpenAI, Ollama, DashScope Bailian coding, and executor apps
• avoid generic map value wrappers in MemoryStateStore internals while preserving the external generic API until upstream V generic map support lands

Validation

• v -cc clang test src/state_store_test.v
• v -cc clang test src/openai_runtime_test.v
• v -cc clang test src/inproc_vjsx_executor_test.v

Note: default tcc-based test runs currently hit the local vjsx divti3.c path issue, so validation used clang.