Add row mask builder APIs for multifile hybrid scan

[mhaseeb123]

Description

Contributes to #22583

This PR adds multifile hybrid scan reader APIs to construct row masks for two-step materialization.

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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📥 Commits

Reviewing files that changed from the base of the PR and between aedc4d2 and d08ba9d.

📒 Files selected for processing (1)

• cpp/tests/io/experimental/hybrid_scan_multifile_filters_test.cpp

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📝 Walkthrough

Summary by CodeRabbit

• Tests
  • Improved validation assertions for row mask handling in hybrid scan operations to ensure accuracy of row group processing.

Walkthrough

Adds <functional> include to the hybrid scan multifile filters test file and strengthens the BuildAllTrueRowMask test by computing expected_num_rows from reader->total_rows_in_row_groups(row_group_indices), asserting mask column size against it, and verifying the host-side count of true values equals expected_num_rows.

Changes

BuildAllTrueRowMask test assertion improvements

Layer / File(s)	Summary
Header addition and row-mask assertion refinement cpp/tests/io/experimental/hybrid_scan_multifile_filters_test.cpp	Adds <functional> include; introduces expected_num_rows derived from total_rows_in_row_groups, uses it to assert mask column size, and adds a host-side count of true values in the BOOL8 mask to validate correctness.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related issues

• [BUG] Hybrid scan must support row masks when reading 2B+ rows #22672: The updated assertions directly validate build_all_true_row_mask() sizing behavior using total_rows_in_row_groups(), which is the row mask handling concern described in this issue.

Possibly related PRs

• rapidsai/cudf#22716: Changes to hybrid scan row-mask sizing and offset computation using total_rows_in_row_groups align directly with the assertion logic added here.
• rapidsai/cudf#22793: Introduces the hybrid_scan_multifile::build_all_true_row_mask API that the updated test assertions here validate.
• rapidsai/cudf#22715: Adds multifile row-group pruning APIs including total_rows_in_row_groups, against which the new test expectations are derived.

Suggested reviewers

• shrshi
• lamarrr
• vuule
• vyasr

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (1 warning, 1 inconclusive)

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.
Title check	❓ Inconclusive	The title refers to adding row mask builder APIs for multifile hybrid scan, but the actual changeset only modifies test assertions and adds an include to a test file, not the APIs themselves.	Clarify whether this PR actually adds the APIs or only adds tests for them. If it only adds tests, consider a title like 'Add tests for multifile hybrid scan row mask builder APIs' or similar.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description states the PR adds multifile hybrid scan reader APIs for row masks, which aligns with the test file changes that validate row mask construction functionality.
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

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[Matt711]

Thanks, nothing substantive to add here. I should probably be using these APIs in #22857

[mhaseeb123]

/merge