feat: allow complex JSON values in request _meta

Closes #1910

`_meta` was typed `Record<string, string>` end to end in the Inspector —
on disk as a `{ key, value }[]` pair array, in the Server Settings form as
key/value rows, and in every request verb's per-call metadata parameter.
Nothing in the MCP spec restricts a `_meta` value to a string, and the SDK
models the field as a `z.looseObject`, so the restriction was ours alone.

Widen the whole path to `RequestMetadata` (`Record<string, JsonValue>`):

- Per-server default metadata is a JSON **object** on disk and in memory.
  The pre-#1910 pair array is still read (`normalizeStoredMetadata`) so an
  existing `mcp.json` keeps working; it is never written back, so a save
  migrates the entry.
- The Server Settings → Request Metadata section is a JSON editor
  (`JsonObjectInput`) instead of key/value rows — rows cannot express an
  object value. It keeps the typed text as the display source and emits
  only parsed JSON, the same draft split `SchemaJsonField` uses, so the box
  stays typeable; invalid text is flagged inline and not applied.
- The CLI stops flattening `--metadata` / `--tool-metadata` through
  `metaValueToString`. Those values were already JSON-parsed, so
  `trace={"id":"abc"}` used to reach the wire as the *string* `{"id":"abc"}`
  and now reaches it as an object.
- `servers/show` redaction replaces a sensitive key's whole value rather
  than a string, so a structured value cannot leak through its members.
- `progressTokenOf` narrows the reserved `progressToken` key back to
  `string | number` before it reaches the SDK; any other JSON still travels
  inside `_meta` but wires up no progress callback.

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/react/usePagedPrompts.ts

@@ -1,5 +1,6 @@
 import { useState, useEffect, useCallback } from "react";
 import type { InspectorClientProtocol } from "../mcp/inspectorClientProtocol.js";
+import type { RequestMetadata } from "../mcp/types.js";
 import type {
   PagedPromptsState,
   PagedPromptsStateEventMap,
@@ -23,7 +24,7 @@ export interface UsePagedPromptsResult {
   error: Error | null;
   loadPage: (
     cursor?: string,
-    metadata?: Record<string, string>,
+    metadata?: RequestMetadata,
   ) => Promise<LoadPageResult>;
   clear: () => void;
 }
@@ -84,7 +85,7 @@ export function usePagedPrompts(
   const loadPage = useCallback(
     async (
       cursor?: string,
-      metadata?: Record<string, string>,
+      metadata?: RequestMetadata,
     ): Promise<LoadPageResult> => {
       if (!pagedPromptsState || !client) {
         return { prompts: [], nextCursor: undefined };