Align `median` and `percentile_cont` implementations (preserve Decimal in `percentile_cont`, alias `median`)

[CuteChuanChuan]

Is your feature request related to a problem or challenge?

In #21988, we fixed median to return Float64 for integer inputs (avoiding truncation) while preserving Decimal and floating-point types as-is. This means after #21988:

• median(int_col) → Float64
• median(float_col) → Float32/Float64 (preserved)
• median(decimal_col) → Decimal* (preserved)

However, percentile_cont — which is functionally equivalent to median when the percentile is 0.5 — casts all Numeric inputs (including Decimal) to Float64:
https://github.com/apache/datafusion/blob/main/datafusion/functions-aggregate/src/percentile_cont.rs#L143-L157

So median(decimal_col) and percentile_cont(0.5) WITHIN GROUP (ORDER BY decimal_col) return different types for the same input — Decimal* vs Float64. As @Jefffrey noted in #21988, this divergence between two essentially identical functions is surprising.

Describe the solution you'd like

Per @alamb's suggestion in #21988 (comment), align in two phases:

Phase 1 — extend percentile_cont to preserve Decimal (and Float32)

Update PercentileCont::new so the value-argument coercion accepts Decimal and floating-point types directly, matching median's post-#21988 behavior. The percentile argument (the 0.5) keeps its current Float64 coercion.

After this change:

• percentile_cont(0.5) WITHIN GROUP (ORDER BY decimal_col) → Decimal* (preserved)
• percentile_cont(0.5) WITHIN GROUP (ORDER BY int_col) → Float64
• percentile_cont(0.5) WITHIN GROUP (ORDER BY float_col) → Float32/Float64 (preserved)

Phase 2 — fold median into percentile_cont as an alias

Once the signatures and return types align, remove the standalone Median implementation and register median as an alias of percentile_cont with 0.5 as the percentile.

Describe alternatives you've considered

Make median match percentile_cont's current Float64-everywhere behavior — discussed in #21988 and not chosen because casting Decimal to Float64 discards the exact-precision guarantee users opt into when picking Decimal.

Additional context

• Discussion: fix: median returns Float64 for integer inputs to avoid truncation #21988 (review)
• Original median fix: fix: median returns Float64 for integer inputs to avoid truncation #21988 (closes bug: Median() truncates integers #19536 )

[CuteChuanChuan]

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