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/tui/__tests__/App.test.tsx

@@ -340,7 +340,7 @@ function oneEmaHttp(): Record<string, TuiServer> {
       config: { type: "streamable-http", url: "http://localhost:8080/mcp" },
       settings: {
         requestTimeout: 0,
-        metadata: [],
+        metadata: {},
         headers: [],
         env: [],
         roots: [],

clients/tui/__tests__/helpers/oauth-test-fakes.ts

@@ -19,7 +19,7 @@ export function makeFakeServerSettings(
 ): InspectorServerSettings {
   return {
     headers: [],
-    metadata: [],
+    metadata: {},
     env: [],
     connectionTimeout: 0,
     requestTimeout: 0,

clients/tui/src/App.tsx

@@ -291,13 +291,9 @@ function App({
           transport: createTransportNode,
           logger: getTuiLogger(),
         };
-        const defaultMetadata = savedSettings?.metadata
-          ? Object.fromEntries(
-              savedSettings.metadata
-                .filter((m) => m.key.trim() !== "")
-                .map((m) => [m.key, m.value]),
-            )
-          : undefined;
+        // Per-server default `_meta` is already a JSON object (#1910) — no
+        // pair-array flattening left to do; `{}` means "no defaults".
+        const defaultMetadata = savedSettings?.metadata;
         const clientAuthOptions = buildRunnerClientAuthOptions(
           clientConfig,
           savedSettings,