fix: Raise if n or fraction cannot be cast in sample() for DataFrame and Expr

[etiennebacher]

Fixes #21521

Before

import polars as pl

df = pl.DataFrame({"a": [1], "foo": ["x"]})

### df
df.sample(fraction="foo")
# shape: (0, 2)
# ┌─────┬─────┐
# │ a   ┆ foo │
# │ --- ┆ --- │
# │ i64 ┆ str │
# ╞═════╪═════╡
# └─────┴─────┘
df.sample(n="foo")
shape: (0, 2)
# ┌─────┬─────┐
# │ a   ┆ foo │
# │ --- ┆ --- │
# │ i64 ┆ str │
# ╞═════╪═════╡
# └─────┴─────┘

### expr
df.select(pl.col("a").sample(fraction="foo"))
# shape: (0, 1)
# ┌─────┐
# │ a   │
# │ --- │
# │ i64 │
# ╞═════╡
# └─────┘

df.select(pl.col("a").sample(n="foo"))
# shape: (0, 1)
# ┌─────┐
# │ a   │
# │ --- │
# │ i64 │
# ╞═════╡
# └─────┘

After

import polars as pl

df = pl.DataFrame({"a": [1], "foo": ["x"]})

### df
df.sample(fraction="foo")
# polars.exceptions.InvalidOperationError: conversion from `str` to `f64` failed in column 'frac' for 1 out of 1 values: ["foo"]
df.sample(n="foo")
# polars.exceptions.InvalidOperationError: conversion from `str` to `u32` failed in column '' for 1 out of 1 values: ["foo"]

### expr
df.select(pl.col("a").sample(fraction="foo"))
# polars.exceptions.InvalidOperationError: conversion from `str` to `f64` failed in column 'foo' for 1 out of 1 values: ["x"]
df.select(pl.col("a").sample(n="foo"))
# polars.exceptions.InvalidOperationError: conversion from `str` to `u32` failed in column 'foo' for 1 out of 1 values: ["x"]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.63%. Comparing base (5cec049) to head (44c249a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #22310   +/-   ##
=======================================
  Coverage   80.63%   80.63%           
=======================================
  Files        1677     1677           
  Lines      222209   222209           
  Branches     2801     2801           
=======================================
+ Hits       179178   179185    +7     
+ Misses      42364    42356    -8     
- Partials      667      668    +1     

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