improved BigDecimal parsing performance

[wenshao]

• Use bit-shift operations for faster digit parsing
• Simplify overflow detection to single check after digit loop
• Handle exponent with combined scale to avoid string-based parsing

What this PR does / why we need it?

Summary of your change

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• Made sure tests are passing and test coverage is added if needed.
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• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.