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/index.ts

@@ -1,9 +1,14 @@
-export { getServerAdapter, listAdapterModels, listServerAdapters } from "./registry.js";
+export { getServerAdapter, listAdapterModels, listServerAdapters, findServerAdapter } from "./registry.js";
 export type {
   ServerAdapterModule,
   AdapterExecutionContext,
   AdapterExecutionResult,
   AdapterInvocationMeta,
+  AdapterEnvironmentCheckLevel,
+  AdapterEnvironmentCheck,
+  AdapterEnvironmentTestStatus,
+  AdapterEnvironmentTestResult,
+  AdapterEnvironmentTestContext,
   AdapterSessionCodec,
   UsageSummary,
   AdapterAgent,