fix: preserve floating-point percentile interpolation precision

[aoto-tech]

Which issue does this PR close?

• Closes percentile_cont truncates Float64 interpolation weights to six decimal places #25266.

Rationale for this change

For floating-point inputs, percentile_cont currently truncates the interpolation weight to six decimal places. A small but nonzero percentile such as 2.5e-7 therefore returns the lower input value instead of an interpolated result.

What changes are included in this PR?

I removed the integer quantization from the float path and apply the f64 interpolation fraction directly. Decimal interpolation still uses the existing fixed precision; I renamed the constant to make that boundary explicit.

I also use separately weighted terms when finite endpoints have opposite signs. The usual difference-based form stays in place otherwise, but it would overflow before applying the fraction for a range such as -f64::MAX..=f64::MAX.

What is the testing strategy for this PR?

I added a unit test that exercises the reported sub-millionth weight through calculate_percentile for Float16, Float32, and Float64. There is also a focused test for the opposite-extreme overflow path, plus an sqllogictest covering the three float types. Three existing expected values change because they previously included the six-decimal truncation error.

I ran:

• cargo test --locked -p datafusion-functions-aggregate --lib -j 4 (234 passed)
• cargo test --locked -j 4 --test sqllogictests -- aggregate.slt
• cargo fmt --all -- --check

I also ran Clippy for all targets in datafusion-functions-aggregate. With Rust 1.97 I had to allow the pre-existing unnecessary_semicolon warning in datafusion/common/src/rounding.rs:257; otherwise Clippy stops in that dependency before reaching this crate.

Are there any user-facing changes?

Yes. percentile_cont now preserves the full interpolation weight for floating-point inputs. There is no public API change.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.91%. Comparing base (85d4cbb) to head (9568827).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #25275   +/-   ##
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  Coverage   81.91%   81.91%           
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  Files        1134     1134           
  Lines      425631   425673   +42     
  Branches   425631   425673   +42     
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+ Hits       348647   348685   +38     
- Misses      56304    56308    +4     
  Partials    20680    20680           

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