fix: reject non-finite numbers before they reach _meta

`StrictJsonValue` removed `undefined`, but the type cannot express the rest of
the gap: TypeScript has one `number`, and JSON has no encoding for `NaN` or
`±Infinity`, so `JSON.stringify` rewrites them to `null`.

This is reachable from ordinary input, not a careless caller.
`JSON.parse('{"a":1e400}')` returns `Infinity` on overflow — so text a user
types parses cleanly, satisfies the type, and reaches the server as `null`
while the editor still shows `1e400`.

`isSerializableJson` in `core/json` is the runtime half of `StrictJsonValue`,
applied at both boundaries that turn text into metadata:

- the editor's draft parser, which now reports "Numbers must be finite" inline
  and withholds the value rather than emitting one it cannot send;
- the CLI's `--metadata` / `--tool-metadata` parser, which throws. Silently
  sending `null` would be wrong, and so would falling back to the literal
  string — neither is what was asked for.

Tests cover the validator directly (including that `JSON.stringify` really does
write `null`, which is the reason it matters), the editor parser, and the CLI
flag.

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__/metadata.test.ts

@@ -880,6 +880,37 @@ describe("Metadata Tests", () => {
       }
     });
 
+    it.each([
+      ["an overflowing literal", "n=1e400"],
+      ["a negative overflow", "n=-1e400"],
+      ["one nested in an object", 'o={"a":1e400}'],
+    ])("rejects %s rather than sending null", async (_label, pair) => {
+      // `JSON.parse` accepts these and yields ±Infinity, which
+      // `JSON.stringify` writes as `null` — so accepting the flag would
+      // transmit a value the user did not ask for.
+      const server = createTestServerHttp({
+        serverInfo: createTestServerInfo(),
+        tools: [createEchoTool()],
+      });
+      try {
+        await server.start();
+        const result = await runCli([
+          server.url,
+          "--cli",
+          "--method",
+          "tools/list",
+          "--metadata",
+          pair,
+          "--transport",
+          "http",
+        ]);
+        expect(result.exitCode).not.toBe(0);
+        expect(`${result.stderr}${result.stdout}`).toMatch(/finite/i);
+      } finally {
+        await server.stop();
+      }
+    });
+
     it("should handle metadata parsing validation", async () => {
       const server = createTestServerHttp({
         serverInfo: createTestServerInfo(),