[opt](parquet) accelerate fixed binary decimal decoding

[Gabriel39]

Proposed changes

• Add a same-scale fast path for fixed-length binary decimal decoding based on physical width.
• Decode full-width big-endian values directly and preserve sign extension for narrow inputs.
• Keep target-precision validation before narrowing and preserve strict/non-strict conversion behavior.
• Add boundary, sign-extension, and narrow-source-domain coverage.

Dependency

This is a stacked PR based on #66360.

Testing

• Existing targeted decimal SerDe unit tests.

[hello-stephen]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?