Add the predicate in physical plan explain output

[Matt711]

Description

Includes the predicate in the physical plan explain output for ConditionalJoin, Filter, and Scan. Purely an improvement; helps make studying plan easier.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Configuration used: Path: .coderabbit.yaml

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Run ID: 8a38efe7-0501-4060-b494-6fa223685a18

📥 Commits

Reviewing files that changed from the base of the PR and between 14e9e9c and 46e219d.

📒 Files selected for processing (2)

• python/cudf_polars/cudf_polars/streaming/explain.py
• python/cudf_polars/tests/streaming/test_explain.py

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Explain output now shows filter, scan, and conditional join predicates in a clearer, human-readable format.
  • Structured expression output now preserves more readable predicate representations.

• Tests

  • Added coverage for predicate formatting, including simple comparisons and nested logical expressions.
  • Added explain-plan coverage for conditional joins, including warning and output verification.

Walkthrough

The streaming explain formatter now renders predicates for conditional joins, filters, and scans, and the serializer matches Col and Literal directly. Tests cover predicate string formatting and conditional join explain output.

Changes

Streaming explain predicate rendering

Layer / File(s)	Summary
Predicate formatting and explain headers python/cudf_polars/cudf_polars/streaming/explain.py	Adds predicate string rendering for expression nodes, includes predicates in ConditionalJoin and Filter headers, extends Scan labels with scan predicates, and updates expression serialization to match Col and Literal directly.
Predicate rendering tests python/cudf_polars/tests/streaming/test_explain.py	Adds coverage for _predicate_to_str on columns, binary comparisons, and nested logical expressions, and checks conditional join explain output with the rendered predicate and warning.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: adding predicate details to physical plan explain output.
Description check	✅ Passed	The description matches the implemented changes and accurately describes the updated explain output and tests.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[mroeschke]

optional: Would be cool to make e.g. str(plc.binaryop.BinaryOperator.EQUAL) return "==".

[Matt711]

Yeah, it does look nice. But I wonder if it's important not to lose the information already provided like that it's a value from an enum?

In [4]: str(plc.binaryop.BinaryOperator.EQUAL)
Out[4]: '<binary_operator.EQUAL: 21>'

[mroeschke]

Making __repr__ preserve this information while __str__ representing the symbolic version of the comparison could be a distinction (IIRC I think e.g. operator.eq does something similar), but overall NBD

[Matt711]

/merge