fix: median returns Float64 for integer inputs to avoid truncation

[CuteChuanChuan]

Which issue does this PR close?

• Closes bug: Median() truncates integers #19536.

Rationale for this change

MEDIAN over integer columns previously returned the input integer type, which truncates the .5 produced when averaging the two middle values of an even-length input.

For example:

SELECT median(v) FROM (VALUES (arrow_cast(-128, 'Int8')),
                              (arrow_cast(127,  'Int8'))) AS t(v);
-- Old: 0    (truncated from -0.5)
-- New: -0.5

What changes are included in this PR?

• Replace Median's Signature::numeric with Signature::one_of containing three coercible arms:
  • Decimal → preserved exactly
  • Float → preserved exactly
  • Integer → coerced implicitly to Float64
• Update SLT and insta snapshot expectations affected by the new return type (column type indicators I → R, previously truncated values such as -17 → -17.5).

Are these changes tested?

Yes. Includes:

• Existing SLT cases for median over Int8/16/32/64 and UInt8/16/32/64
• Boundary cases that previously truncated.
• insta snapshot tests.

Are there any user-facing changes?

Yes — this is a breaking change for median(<integer_column>): Return type changes from the input integer type to Float64.