[Task] Document the WASM host ABI and add a gateway-level reference plugin test #7563

Description

Make the wasm-api/wasm-base contract verifiable with a versioned ABI specification and a real gateway integration example.

The modules currently have Rust and TinyGo fixtures demonstrating shenyu.get_args/put_result, lifecycle exports and shared-memory communication. The Go ABI helper explicitly lives under test sources for internal test use. Repository searches did not find a concrete WASM business plugin or WASM integration/e2e suite exercising these abstractions through bootstrap.

This does not imply there is no external ShenYu WASM documentation; the missing evidence is a versioned contract and gateway-level test alongside the current runtime implementation. It also does not require the abstract libraries themselves to have an independent starter.

Task List

• Specify import namespace/signatures, required versus optional exports, pointer/length conventions, serialization, buffer-too-small behavior, result/error codes and ABI compatibility/versioning.
• State which build targets/toolchains are tested; distinguish TinyGo fixtures from support for arbitrary Go runtimes.
• Add a small reference plugin that is actually registered in a gateway test.
• Exercise selector/rule configuration delivery, real HTTP request data, a guest decision/result and chain behavior.
• Exercise missing hooks, configuration updates/deletes, concurrency isolation and runtime failures using actual Wasm fixtures.
• Document reproducible fixture builds and the integration-test command.

Additional context

ShenYu: 2.7.2-SNAPSHOT, commit 2dff2730ff399635b2bc657e21a903f391fb9df8; Java 17; Chicory 1.7.3.

The existing 24 tests across these modules passed locally, along with Checkstyle. They establish basic Rust/TinyGo communication and matching paths, not the full gateway lifecycle.

Related historical tasks: #4492, #4612, #5427. Runtime migration: #6353.