fix: disallow null seed for gen_rng

[poppingmoon]

What

乱数関連の関数の引数の型を狭くし、 Math:gen_rng の seed に null が与えられたときに AiScriptRuntimeError が発生するようにしました。

Why

Fix #1012

Additional info (optional)

PRの宛先のブランチがわからなかったので適当に変更していただければと思います。

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/utils/random/chacha20.ts	57.14%	3 Missing ⚠️
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Files with missing lines	Coverage Δ
src/interpreter/lib/math.ts	64.64% <100.00%> (ø)
src/utils/random/genrng.ts	53.65% <100.00%> (+1.57%)	⬆️
src/utils/random/chacha20.ts	88.81% <57.14%> (+7.11%)	⬆️

... and 28 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.