fix: avoid Float64 percentile_cont interpolation overflow

[aoto-tech]

Which issue does this PR close?

• Closes percentile_cont overflows while interpolating finite Float64 values #25267.

Rationale for this change

percentile_cont can return Infinity for finite Float64 inputs when upper - lower overflows, even though the interpolated result is finite. I hit this with [-DBL_MAX, DBL_MAX] at 0.5; the median should be 0.

What changes are included in this PR?

I keep the current difference-based formula when the subtraction is finite. If both endpoints are finite but their difference overflows, I switch to weighting each endpoint separately. That avoids the infinite intermediate without changing the existing Float16 or non-finite-input paths.

I also added a direct unit test for the overflow branch with symmetric, asymmetric, and quarter-percentile cases, plus an end-to-end SQL test for the reported query.

What is the testing strategy for this PR?

The new unit test failed with inf != 0.0 before the fix.

I ran:

• cargo test --profile ci -p datafusion-functions-aggregate --lib (233 passed)
• the new aggregate.slt query through the sqllogictest runner (passed)
• cargo clippy --profile ci -p datafusion-functions-aggregate --all-targets --all-features --no-deps -- -D warnings
• cargo fmt --all -- --check

Are there any user-facing changes?

Yes. percentile_cont now returns a finite result when finite Float64 endpoints have an overflowing difference but a finite interpolation result. There is no 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 (f832f61).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #25274   +/-   ##
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  Coverage   81.91%   81.91%           
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  Files        1134     1134           
  Lines      425631   425653   +22     
  Branches   425631   425653   +22     
=======================================
+ Hits       348647   348667   +20     
  Misses      56304    56304           
- Partials    20680    20682    +2     

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