[refactor](cloud) improve FDB transaction approximate size calculation accuracy

[w41ter]

Cherry-pick #60351 to branch-4.0

Original PR:

• refactor improve FDB transaction approximate size calculation accuracy

Refactored the approximate size calculation in the Transaction class to align with FoundationDB's actual implementation in ReadYourWrites.actor.cpp.

[Thearas]

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Please clearly describe your PR:

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?

[dataroaring]

run buildall

[doris-robot]

Cloud UT Coverage Report

Increment line coverage 100.00% (36/36) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	79.26% (1789/2257)
Line Coverage	64.84% (31838/49101)
Region Coverage	65.46% (15887/24269)
Branch Coverage	55.99% (8436/15068)