fix(connectors): reject undersized RDMA object bounds at registration

[ketor]

Summary

• expose the active transport's resolved logical object ceiling through a read-only C API
• fail HiCache host-pool and vLLM cache-group registration when actual object geometry exceeds that ceiling
• preserve explicit operator limits, TCP behavior, wire formats, and runtime transport rejection semantics
• keep the shared vLLM hot-config watcher alive when a partially constructed client is closed

Validation

• configured and built with RDMA and tests enabled
• C API suite: 11 passed
• common block-size preflight suite: 8 passed
• vLLM connector suite: 15 passed
• Python plugin CTest passed
• hardware-backed RDMA smoke covered default, explicit, and payload-clamped bounds
• HiCache registration smoke covered oversize rejection and equality acceptance with independent component geometry
• full SGLang and vLLM integration runs reached readiness with the new registration checks; fresh reader processes completed successful external GETs and correctness checks

RDMA loopback tests remain self-skipping on hosts without an HCA; the hardware-backed checks above were run separately.