Address review: justify/refine SDK-gap casts (as unknown as)

Per human review on #1688: for each `as unknown as` in the PR, either refactor
or justify with a comment naming the SDK gap (so it can inform SDK-side type
improvements).

- Sampling/elicit receiver-task returns: route through a typed `CreateTaskResult`
  so the object shape IS checked; the remaining cast to CreateMessageResult /
  ElicitResult is the honest SDK gap (the 2-arg setRequestHandler overload types
  the handler return as the spec result only, not the task-augmented variant).
- `_requestHandlers` / `_progressHandlers` private-field casts: comments now name
  the exact missing public API (opt a handler out of result validation; keep a
  progress subscription alive across a resolved request).
- test-server `_requestHandlers` cast: same SDK gap on the server side, noted.
- serverList object->Record casts (pre-existing): documented as plain structural
  widening (no index signature -> TS requires the `unknown` step), not an SDK
  workaround.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test-servers/src/composable-test-server.ts

@@ -1215,6 +1215,12 @@ function wireTaskHandlers(
   taskTools: Map<string, TaskToolDefinition>,
 ): void {
   const lowLevel = mcpServer.server;
+  // SDK gap (server side): `Server` exposes no public API to override an already
+  // registered handler or to send a `tools/call` response that bypasses its
+  // result-schema validation, so we reach the private `_requestHandlers` map via
+  // a narrowed cast (`RawHandlerHost`). Mirrors the client-side bypass in
+  // `core/mcp/inspectorClient.ts`; both go away when the SDK models task-augmented
+  // results natively.
   const registry = (lowLevel as unknown as RawHandlerHost)._requestHandlers;
   const sdkToolsCall = registry.get("tools/call");
   // Map each created task back to the tool that owns it, so tasks/get and