fix(server): accept empty {} annotations in tool() overload

[AP3X-Dev]

Summary

server.tool(name, description, schema, {}, callback) registers successfully but throws typedHandler is not a function at first call.

The overload parser at src/server/mcp.ts:993 uses isZodRawShapeCompat() to disambiguate between a Zod raw shape and a ToolAnnotations object in the slot after the schema. isZodRawShapeCompat({}) returns true (intentionally — to accept tools with no parameters in the schema slot), so an empty annotations object passed after a populated schema is misclassified as a second schema, the annotations branch is skipped, and callback = rest[0] ends up as {} instead of the function.

At dispatch time, executeToolHandler does:

```ts
const typedHandler = handler;
return await Promise.resolve(typedHandler(args, extra));
```

…which throws TypeError: typedHandler is not a function.

Root cause

src/server/mcp.ts:1021 (pre-fix):

```ts
if (rest.length > 1 && typeof rest[0] === 'object' && rest[0] !== null && !isZodRawShapeCompat(rest[0])) {
annotations = rest.shift() as ToolAnnotations;
}
```

For empty {}, !isZodRawShapeCompat({}) is false, so annotations are not consumed. The handler slot then receives {}.

Fix

Once the schema slot has been filled above, an empty object in the next slot is unambiguously annotations (a populated raw shape would have at least one Zod value). The annotations check now accepts empty objects:

```ts
if (
rest.length > 1 &&
typeof rest[0] === 'object' &&
rest[0] !== null &&
(Object.keys(rest[0] as object).length === 0 || !isZodRawShapeCompat(rest[0]))
) {
annotations = rest.shift() as ToolAnnotations;
}
```

Non-empty objects retain their existing protective check (a populated Zod raw shape in the annotations slot stays an error rather than being silently consumed as annotations).

Test plan

• New regression test in test/server/mcp.test.ts covering tool(name, desc, schema, {}, cb) — registers, lists, and calls successfully (passes against both Zod v3 and Zod v4 fixtures).
• Full test/server/mcp.test.ts suite passes (234/234).
• npm run typecheck passes.
• npx prettier --check passes on touched files.
• npx eslint src/server/mcp.ts passes.

Real-world impact

Any tool registered with server.tool(name, desc, schema, {}, cb) is unreachable. Encountered this in a downstream MCP server where 6 write-tool registrations all used {} as a placeholder for "no annotations to declare yet" — every write call surfaced as typedHandler is not a function while reads (which used populated annotations) worked.

Affects v1.x line. Not present on main — the 2.0 rewrite uses a different dispatch path.

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