Add adapter environment testing infrastructure

Introduce testEnvironment() on ServerAdapterModule with structured
pass/warn/fail diagnostics for all four adapter types (claude_local,
codex_local, process, http). Adds POST test-environment endpoint,
shared types/validators, adapter test implementations, and UI API
client. Includes asset type foundations used by related features.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/adapters/http/index.ts

@@ -1,9 +1,11 @@
 import type { ServerAdapterModule } from "../types.js";
 import { execute } from "./execute.js";
+import { testEnvironment } from "./test.js";
 
 export const httpAdapter: ServerAdapterModule = {
   type: "http",
   execute,
+  testEnvironment,
   models: [],
   agentConfigurationDoc: `# http agent configuration