Fix TypeScript return types for sum along a dimension

[WenJing95]

Fixes #3202.

Correct the TypeScript overloads for sum(collection, dimension): one-dimensional arrays produce scalars, two-dimensional arrays produce arrays, and Matrix inputs produce Matrix results. Widen number and bigint literals in reductions and add type and runtime regression assertions.

Validation on Windows with Node 22.20.0:

• TypeScript compilation, focused sum tests (16 passing, 2 pending), new regression runtime assertions, lint, and the default Gulp build passed.
• Full source suite: 6,648 passing, 22 pending, and four failures in matrixFromRows, matrixFromColumns, acosh, and asech, identical on develop and this branch.
• npm run test:types: TypeScript compilation passed; the ts-node runtime stage failed on both develop and this branch.