fix: address Copilot review on the _meta widening

- Widen the three `_meta` annotations the bulk pass missed, all of them
  return types rather than identifier declarations: `mergeMeta`,
  `aggregateListParams`, and the `tools/call` params object. The second was
  unsound in a way callers could see — it advertised
  `{ _meta: Record<string, string> }` while the runtime carried nested JSON.

- `normalizeStoredMetadata`: keep an explicit `null` from a legacy pair
  instead of folding it into `""` (only a missing `value` defaults now), and
  store through `Object.defineProperty` so a legacy `__proto__` key becomes an
  own property rather than hitting the prototype setter.

- Drop the unjustified `as unknown as` from the migration test; the legacy
  fixture is untyped and cast once where it crosses into the typed parser.

- The TUI settings test asserted only that the app mounted, so it could not
  have caught a pair array being forwarded as numeric `_meta` keys. The fake
  client now captures its constructor options and the test asserts the nested
  object arrives intact.

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/test/core/mcp/serverList.test.ts

@@ -1424,6 +1424,26 @@ describe("normalizeStoredMetadata (#1910)", () => {
     ).toEqual({ kept: "yes" });
   });
 
+  it("preserves an explicit null in a legacy pair rather than blanking it", () => {
+    // `null` is a legal `_meta` value, so the migration must not fold it into
+    // `""` — only a genuinely absent `value` defaults.
+    expect(normalizeStoredMetadata([{ key: "z", value: null }])).toEqual({
+      z: null,
+    });
+  });
+
+  it("stores a legacy `__proto__` key as an own property", () => {
+    // Plain assignment would hit the prototype setter: the entry would vanish
+    // (and an object value would mutate the prototype) instead of being stored.
+    const out = normalizeStoredMetadata([
+      { key: "__proto__", value: { polluted: true } },
+    ]);
+    expect(Object.hasOwn(out, "__proto__")).toBe(true);
+    expect(out["__proto__"]).toEqual({ polluted: true });
+    expect(Object.getPrototypeOf(out)).toBe(Object.prototype);
+    expect(({} as Record<string, unknown>).polluted).toBeUndefined();
+  });
+
   it("reads a legacy pair with no value as an empty string", () => {
     expect(normalizeStoredMetadata([{ key: "k" }])).toEqual({ k: "" });
   });
@@ -1460,18 +1480,19 @@ describe("normalizeStoredMetadata (#1910)", () => {
   it("migrates a legacy file to the object shape on the next write", () => {
     // Read side accepts the pair array; write side emits only the object, so
     // one round-trip is the migration.
-    const entries = mcpConfigToServerEntries({
+    // The legacy on-disk shape no longer matches `MCPConfig` — that is what
+    // the test is about — so the fixture is untyped and cast once where it
+    // crosses into the typed parser, the same boundary a real file crosses.
+    const legacyFile: unknown = {
       mcpServers: {
         legacy: {
           type: "streamable-http",
           url: "https://x.test/mcp",
-          metadata: [{ key: "tenant", value: "acme" }] as unknown as Record<
-            string,
-            never
-          >,
+          metadata: [{ key: "tenant", value: "acme" }],
         },
       },
-    });
+    };
+    const entries = mcpConfigToServerEntries(legacyFile as MCPConfig);
     expect(entries[0]?.settings?.metadata).toEqual({ tenant: "acme" });
     const round = serverEntriesToMcpConfig(entries);
     expect(round.mcpServers.legacy?.metadata).toEqual({ tenant: "acme" });