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>

clients/web/src/components/groups/ServerSettingsModal/ServerSettingsModal.stories.tsx

@@ -14,7 +14,7 @@ const initialSettings: InspectorServerSettings = {
     { key: "X-Request-Id", value: "req-456" },
   ],
   env: [],
-  metadata: [{ key: "userId", value: "user-789" }],
+  metadata: { userId: "user-789", client: { name: "inspector" } },
   connectionTimeout: 30000,
   requestTimeout: 60000,
   taskTtl: 60000,
@@ -74,7 +74,7 @@ export const EmptySettings: Story = {
     settings: {
       headers: [],
       env: [],
-      metadata: [],
+      metadata: {},
       connectionTimeout: 30000,
       requestTimeout: 60000,
       taskTtl: 60000,
@@ -96,7 +96,7 @@ export const StdioServer: Story = {
         { key: "API_KEY", value: "abc-123" },
         { key: "LOG_LEVEL", value: "debug" },
       ],
-      metadata: [],
+      metadata: {},
       connectionTimeout: 0,
       requestTimeout: 0,
       taskTtl: 60000,