standardized runtime invocation contract + conformance tests

[ashupednekar]

Summary

Define and enforce a runtime contract for sync and streaming invocation paths.

Motivation

Language runtimes currently differ in framing/error behavior, creating compatibility drift.

Proposal

• Standard request/response envelope (headers, metadata, payload, error shape).
• Stream chunk protocol for SSE/WS/NATS responses.
• Standard timeout/cancellation propagation model.
• Conformance tests for each runtime.

Acceptance criteria

• Contract is documented in one canonical spec.
• All runtimes pass conformance tests.
• Ingestor behavior is consistent across languages.