feat: decimal support for percentile_cont and median

[theirix]

Which issue does this PR close?

• Closes Align median and percentile_cont implementations (preserve Decimal in percentile_cont, alias median) #22042.

Rationale for this change

UDF percentile_cont uses floating-point arithmetic to find a percentile, which doesn't work well for decimals. The median UDF already supports decimals. Let's implement decimals in percentile_cont and build median on top of it.

A follow-up to #21988, refines #23954.

What changes are included in this PR?

• Exact decimal support for percentile_cont, without float coercion
• Integer support for percentile_cont (backported from median UDF)
• Linear interpolation for both float (old code), decimal and integer code paths (new code)
• Implement median on top of percentile_cont UDF by prepending a 0.5f argument
• Add SLTs to verify code
• Make percentile_cont(NULL) return NULL (see changes below), under a new SLT test case

Are these changes tested?

• Extend SLTs
• Unit test to verify integer inputs (cannot be done in SLT)
• Unit tests for percentile_cont

Are there any user-facing changes?

• percentile_cont(NULL) now returns a NULL type instead of a Float64 type, aligning with other aggregate UDFs

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.02439% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.25%. Comparing base (bbf739a) to head (df896d2).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...afusion/functions-aggregate/src/percentile_cont.rs	88.29%	9 Missing and 15 partials ⚠️
datafusion/functions-aggregate/src/median.rs	92.68%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24419      +/-   ##
==========================================
+ Coverage   81.23%   81.25%   +0.01%     
==========================================
  Files        1111     1112       +1     
  Lines      390208   390464     +256     
  Branches   390208   390464     +256     
==========================================
+ Hits       316990   317257     +267     
+ Misses      54591    54556      -35     
- Partials    18627    18651      +24     

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[theirix]

CC @neilconway

[Jefffrey]

Integer support for percentile_cont (backported from median UDF)

this doesnt sound right, since this is continuous and not discrete so it must always be a float (or decimal)

[Jefffrey]

i do wonder if we should implement simplify for median to make it an unconditional rewrite to percentile_cont

[Jefffrey]

Suggested change

	Coercion::new_implicit(
	TypeSignatureClass::Native(logical_float64()),
	vec![TypeSignatureClass::Numeric],
	NativeType::Float64,
	),
	Coercion::new_implicit_native(
	logical_float64(),
	vec![TypeSignatureClass::Numeric],
	),

• new simpler API from feat: add implicit native coercion constructor #24282

[Jefffrey]

i think its intentional to check the percentile up here, so that we still error on invalid percentiles even if input is null

[Jefffrey]

are integer types supported in the signature? is this code being reached?