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/cli/__tests__/servers-list.test.ts

@@ -219,10 +219,14 @@ describe("showServerEntry / servers/show", () => {
         { key: "Authorization", value: "Bearer x" },
         { key: "X-Custom", value: "ok" },
       ],
-      metadata: [
-        { key: "Authorization", value: "Bearer meta" },
-        { key: "X-Custom", value: "ok" },
-      ],
+      metadata: {
+        Authorization: "Bearer meta",
+        "X-Custom": "ok",
+        // A structured value under a sensitive key: the redaction must replace
+        // the whole value, not walk into it (#1910).
+        "X-Api-Key": { primary: "sk-live-1", fallback: "sk-live-2" },
+        nested: { keep: true },
+      },
       env: [
         { key: "TOKEN", value: "secret" },
         { key: "", value: "still-secret" },
@@ -242,10 +246,13 @@ describe("showServerEntry / servers/show", () => {
       { key: "Authorization", value: "[redacted]" },
       { key: "X-Custom", value: "ok" },
     ]);
-    expect(sanitized.metadata).toEqual([
-      { key: "Authorization", value: "[redacted]" },
-      { key: "X-Custom", value: "ok" },
-    ]);
+    expect(sanitized.metadata).toEqual({
+      Authorization: "[redacted]",
+      "X-Custom": "ok",
+      "X-Api-Key": "[redacted]",
+      // A structured value under a non-sensitive key survives intact.
+      nested: { keep: true },
+    });
     expect(sanitized.env).toEqual([
       { key: "TOKEN", value: "[redacted]" },
       { key: "", value: "[redacted]" },