fix: Support varying quantile values per group in group_by aggregation

[wtn]

Fixes #20951.

When using group_by() with a quantile parameter that varies per group (e.g., pl.col.quantile.first()), all groups incorrectly received the same quantile value instead of each group using its own.

Reproduction

df = pl.DataFrame({
    "value": [1, 2, 1, 2],
    "quantile": [0, 0, 1, 1],
})
df.group_by("quantile").agg(pl.col("value").quantile(pl.col("quantile").first()))
# Expected: quantile=0 -> 1.0, quantile=1 -> 2.0
# Actual: both groups returned 1.0

Cause

AggQuantileExpr::evaluate_on_groups() always called get_quantile() which evaluates the quantile expression against the full dataframe, returning a single scalar. This worked for literal quantile values but failed when the quantile expression varied per group (e.g., first() aggregation).

Fix

Added agg_varying_quantile which accepts a slice of quantile values (one per group) and computes quantile per group using the existing aggregation helpers.

polars-core changes:

• Added agg_helper_idx_on_all_with_idx and _agg_helper_slice_with_idx helpers that pass the group index to closures
• Added agg_varying_quantile_generic that iterates over groups with their corresponding quantile values
• Added agg_varying_quantile methods to Float32Chunked, Float64Chunked, integer ChunkedArray, Series, and Column

polars-expr changes:

• AggQuantileExpr::evaluate_on_groups() now detects whether the quantile is uniform (literal/scalar) or varies per group, and dispatches to the appropriate path

[codecov]

Codecov Report

❌ Patch coverage is 94.35897% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.47%. Comparing base (8ab1657) to head (bc86c4b).

Files with missing lines	Patch %	Lines
...s-core/src/frame/group_by/aggregations/dispatch.rs	90.56%	5 Missing ⚠️
...polars-core/src/frame/group_by/aggregations/mod.rs	95.45%	4 Missing ⚠️
crates/polars-expr/src/expressions/aggregation.rs	95.45%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25606      +/-   ##
==========================================
- Coverage   79.58%   79.47%   -0.11%     
==========================================
  Files        1743     1743              
  Lines      240439   240618     +179     
  Branches     3038     3038              
==========================================
- Hits       191347   191242     -105     
- Misses      48310    48594     +284     
  Partials      782      782              

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