fix: exclude undefined from _meta values, and cover the JSON worker

- `JsonValue` admits `undefined`, so `RequestMetadata` accepted values that do
  not survive serialization: `JSON.stringify` drops an `undefined` object
  member and rewrites an `undefined` array element to `null`. The type
  contradicted the one promise this payload makes. Added `StrictJsonValue` /
  `StrictJsonObject` and used them for the metadata path.

  Deliberately not a change to the repo-wide `JsonValue`: for tool *arguments*
  `undefined` legitimately means "not supplied". The narrowing is sound
  wherever it lands here, since every one of these values comes from
  `JSON.parse`, which cannot produce `undefined`. The CLI's `parseKeyValuePair`
  needed the same tightening — caught by `clients/cli`'s typecheck, which the
  web `tsc -b` does not cover.

- Nothing asserted the JSON worker's annotations, and the wrapper's own "Not
  valid JSON" message comes from our parser rather than from Ace — so a broken
  worker URL would have left every test passing while the gutter diagnostics
  silently disappeared. Added a Storybook play function that waits for a real
  `session.getAnnotations()` error entry, and verified it fails when the worker
  module URL is pointed at a bad path.

- `aria-describedby` referenced only the error, so a supplied `description` was
  never announced and would have been dropped entirely once the draft went
  invalid. Both ids are referenced now, with the description surviving the
  error.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/utils/jsonObjectDraft.ts

@@ -1,15 +1,21 @@
-import type { JsonObject } from "@inspector/core/json/jsonUtils.js";
+import type { StrictJsonObject } from "@inspector/core/json/jsonUtils.js";
 
 /**
  * What a JSON-object editor's draft text currently means: the object to emit,
  * or why it cannot be emitted.
  */
 export type JsonObjectDraft =
-  | { ok: true; value: JsonObject }
+  | { ok: true; value: StrictJsonObject }
   | { ok: false; error: string };
 
-/** A plain JSON object, i.e. not an array and not a scalar. */
-export function isJsonObject(value: unknown): value is JsonObject {
+/**
+ * A plain JSON object, i.e. not an array and not a scalar.
+ *
+ * The `StrictJsonObject` narrowing is sound because the only caller parses with
+ * `JSON.parse`, whose output cannot contain `undefined` — the value that
+ * separates the strict type from `JsonValue`.
+ */
+export function isJsonObject(value: unknown): value is StrictJsonObject {
   return value !== null && typeof value === "object" && !Array.isArray(value);
 }