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>

core/json/jsonUtils.ts

@@ -14,6 +14,29 @@ export type JsonValue =
 
 export type JsonObject = { [key: string]: JsonValue };
 
+/**
+ * A JSON value that survives serialization unchanged — {@link JsonValue}
+ * without `undefined`.
+ *
+ * `JsonValue` admits `undefined` because it is also used for *arguments*, where
+ * it usefully means "this field was not supplied". That meaning does not carry
+ * to a payload the Inspector promises to transmit verbatim: `JSON.stringify`
+ * drops an `undefined` object member entirely and rewrites an `undefined` array
+ * element to `null`, so a value the type accepted is not the value that arrives.
+ *
+ * Used for `_meta` (#1910), whose whole contract is that any JSON the user
+ * writes reaches the wire as written.
+ */
+export type StrictJsonValue =
+  | string
+  | number
+  | boolean
+  | null
+  | StrictJsonValue[]
+  | { [key: string]: StrictJsonValue };
+
+export type StrictJsonObject = { [key: string]: StrictJsonValue };
+
 /**
  * Widen a typed object to a generic string-keyed record so its keys can be
  * iterated or read/written generically. Many of the project's config/SDK types