Error: namespace must have a "ModuleBlock" body.

the dts will fail when namespaces are nested and declared is a different body.

```ts
export namespace cmath {}
export namespace cmath.delta {}
export namespace cmath.vector2 {}
...
```

This works perfectly fine and a valud ts, and works well with other compilers, this fails with dts with
```
Error: namespace must have a "ModuleBlock" body.

361 | function powerset(arr: T[], k?: number): T[][];
362 | }
> 363 | declare namespace cmath.delta {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

at DeclarationScope.convertNamespace (/Users/softmarshmallow/Documents/Apps/grida/grida/node_modules/.pnpm/tsup@8.5.0_jiti@2.4.2_postcss@8.5.3_typescript@5.8.3_yaml@2.7.0/node_modules/tsup/dist/rollup.js:5749:17)
at Transformer.convertNamespaceDeclaration (/Users/softmarshmallow/Documents/Apps/grida/grida/node_modules/.pnpm/tsup@8.5.0_jiti@2.4.2_postcss@8.5.3_typescript@5.8.3_yaml@2.7.0/node_modules/tsup/dist/rollup.js:5917:15)
at Transformer.convertStatement (/Users/softmarshmallow/Documents/Apps/grida/grida/node_modules/.pnpm/tsup@8.5.0_jiti@2.4.2_postcss@8.5.3_typescript@5.8.3_yaml@2.7.0/node_modules/tsup/dist/rollup.js:5891:23)
at new Transformer (/Users/softmarshmallow/Documents/Apps/grida/grida/node_modules/.pnpm/tsup@8.5.0_jiti@2.4.2_postcss@8.5.3_typescript@5.8.3_yaml@2.7.0/node_modules/tsup/dist/rollup.js:5836:16)
at convert (/Users/softmarshmallow/Documents/Apps/grida/grida/node_modules/.pnpm/tsup@8.5.0_jiti@2.4.2_postcss@8.5.3_typescript@5.8.3_yaml@2.7.0/node_modules/tsup/dist/rollup.js:5827:27)
at Object.transform (/Users/softmarshmallow/Documents/Apps/grida/grida/node_modules/.pnpm/tsup@8.5.0_jiti@2.4.2_postcss@8.5.3_typescript@5.8.3_yaml@2.7.0/node_modules/tsup/dist/rollup.js:6227:29)
at Object.writeFile (/Users/softmarshmallow/Documents/Apps/grida/grida/node_modules/.pnpm/tsup@8.5.0_jiti@2.4.2_postcss@8.5.3_typescript@5.8.3_yaml@2.7.0/node_modules/tsup/dist/rollup.js:6366:55)
at writeFile (/Users/softmarshmallow/Documents/Apps/grida/grida/node_modules/.pnpm/typescript@5.8.3/node_modules/typescript/lib/typescript.js:20370:8)
at printSourceFileOrBundle (/Users/softmarshmallow/Documents/Apps/grida/grida/node_modules/.pnpm/typescript@5.8.3/node_modules/typescript/lib/typescript.js:120018:5)
at emitDeclarationFileOrBundle (/Users/softmarshmallow/Documents/Apps/grida/grida/node_modules/.pnpm/typescript@5.8.3/node_modules/typescript/lib/typescript.js:119911:26)
Error: error occurred in dts build
at Worker. (/Users/softmarshmallow/Documents/Apps/grida/grida/node_modules/.pnpm/tsup@8.5.0_jiti@2.4.2_postcss@8.5.3_typescript@5.8.3_yaml@2.7.0/node_modules/tsup/dist/index.js:1545:26)
at Worker.emit (node:events:518:28)
at MessagePort. (node:internal/worker:268:53)
at [nodejs.internal.kHybridDispatch] (node:internal/event_target:827:20)
at MessagePort. (node:internal/per_context/messageport:23:28)
DTS Build error
 ELIFECYCLE  Command failed with exit code 1.
```

Reproduce the failure with the nested namespace declarations shown in the issue and trace the dts build through DeclarationScope.convertNamespace and Transformer.convertNamespaceDeclaration in tsup's declaration bundling path. Done means the dts build accepts these valid nested namespaces and emits declarations without the ModuleBlock error.

Written by the indexing model from the issue text.