Fix TypeScript Random.fill typed array handling

[Shiven0504]

Summary

• Make TypeScript Random.fill detect BigInt typed arrays by array type instead of array.at(0), so empty typed arrays are still handled correctly.
• Use Math.pow for numeric typed-array bounds so 32-bit arrays use the full 32-bit range instead of JS bit-shift wrapping.
• Add focused coverage for Random.fill availability, all integer typed arrays, and empty arrays.

Fixes #5091.

Tests

• corepack pnpm --filter spacetimedb exec vitest run tests/rng.test.ts
• corepack pnpm --filter spacetimedb build:types
• corepack pnpm --filter spacetimedb exec prettier src/server/rng.ts tests/rng.test.ts --check --ignore-path ../../.prettierignore