feat: Add example rows to ValidationError for all rule failures

[Copilot]

Motivation

Closes #282.

Changes

This PR adds a new option max_failure_examples to the config object. Users can set, for example,

dy.Config.set_max_failure_examples(3)

to show up to three failures for each failing rule whenever validation is run. Examples are shown both for lazy and eager validation.

---

Side note for reviewers: copilot was rather unhelpful here 😅

[Oliver Borchert (borchero)]

• Adjust the format_rule_failures method to optionally include examples. Possibly add a helper method to FailureInfo if that is helpful.
• Do not only provide examples for primary key violations but for any type of validation
• Adjust the Rust plugin s.t. the same information is generated during lazy execution

[Copilot]

• Adjust the format_rule_failures method to optionally include examples. Possibly add a helper method to FailureInfo if that is helpful.
• Do not only provide examples for primary key violations b...

Addressed in commit 462f8ce:

1. format_rule_failures extended: Now accepts an optional examples: dict[str, list[str]] | None parameter. Each value is a list of pre-formatted example row strings. The examples are shown for both schema-level and column-level rules.

2. Examples for all rule types: Added FailureInfo.examples() helper method (the suggested helper on FailureInfo) that computes distinct example rows from all data columns for every failing rule. This is used in both schema.py and collection/collection.py eager validation paths.

3. Lazy execution (Rust plugin): Updated all_rules_required to accept data columns as additional inputs after the rule boolean columns (num_rule_columns kwarg tells it where the split is). For each failing rule, the plugin computes up to 5 distinct example rows using AnyValue::Display. The schema.py lazy path now passes cls.column_names() as data columns.

Example output:

# Eager
1 rules failed validation:
 - 'primary_key' failed for 2 rows with 2 distinct examples: [{'a': 1, 'b': 'x', 'c': '1'}, {'a': 1, 'b': 'y', 'c': '2'}]

# Lazy
1 rules failed validation for schema 'MySchema':
 - 'primary_key' failed for 2 rows with 2 distinct examples: [{'a': 1, 'b': "x", 'c': "1"}, {'a': 1, 'b': "y", 'c': "2"}]

Note: string quotes differ slightly between eager (Python str(dict) → single quotes) and lazy (Rust AnyValue::Display → double quotes), but both are readable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (c79baae) to head (3923110).

Additional details and impacted files

@@            Coverage Diff             @@
##           streamin      #286   +/-   ##
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  Coverage    100.00%   100.00%           
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  Files            56        56           
  Lines          3427      3419    -8     
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- Hits           3427      3419    -8     

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