Fix From BCD scheme validation

[marko1olo]

Summary

• Validate the From BCD encoding scheme before using the lookup table.
• Convert invalid direct-link/API values such as From_BCD('', true, false, 'Nibbles') into a controlled operation error instead of an uncaught TypeError.
• Add operation regression coverage for the empty scheme case.

Fixes #2489

Checks

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

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