Fix To Base radix validation

[marko1olo]

Summary

• Reject fractional radix values before passing them to BigNumber#toString().
• Convert values such as To_Base(2.2) into the existing controlled radix validation error instead of an uncaught BigNumber error.
• Add operation coverage for normal base-2 conversion and the fractional radix regression.

Fixes #2487

Checks

• node --no-warnings --no-deprecation --openssl-legacy-provider tests/operations/index.mjs
• npx eslint src/core/operations/ToBase.mjs tests/operations/index.mjs tests/operations/tests/Base.mjs
• npx grunt configTests
• git diff --check

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[GCHQDeveloper581]

Closing as #2561 enables a better fix, as now provided in #2586