fix(fetch): accept a named interface as request body without a cast

FetchOptions['body'] listed `Record<string, unknown>`, which a named interface
is not assignable to (interfaces carry no implicit index signature), so every
caller passing a typed body had to cast it. Widen the object member to `object`:
named interfaces and arrays now assign directly, `any` is not introduced, and
bare primitives (number/boolean) are still rejected. Runtime is unchanged — the
serializer already narrows the body with its own casts.

Guard it with a type test (src/types.test-d.ts, run under vitest --typecheck):
a named interface must assign to the body type and flow through the method
shortcuts with no cast, while bare primitives stay rejected. Enable typecheck
for the package's vitest project so the assertions are enforced by tsc.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/fetch/vitest.config.ts

@@ -3,5 +3,11 @@ import { defineConfig } from 'vitest/config';
 export default defineConfig({
   test: {
     environment: 'node',
+    // Type tests (*.test-d.ts) are statically analyzed with `tsc --noEmit`
+    // alongside the runtime suite.
+    typecheck: {
+      enabled: true,
+      tsconfig: './tsconfig.src.json',
+    },
   },
 });