Add length check to cudf::strings::is_timestamp logic for literals

[davidwendt]

Description

Adds a length check before comparing a literal within the given format against a string row value in the cudf::strings::is_timestamp() API. This prevents reading past the end of the string. Also added a test for this case as well. The other format specifiers (non-literal) already check the length correctly.

Closes #23660

Checklist

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

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 215888d and bfd09e8.

📒 Files selected for processing (2)

• cpp/src/strings/convert/convert_datetime.cu
• cpp/tests/strings/datetime_tests.cpp

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved timestamp validation for formats containing trailing literal characters.
  • Prevented invalid or incomplete timestamp inputs from being incorrectly accepted or causing unsafe reads.

Walkthrough

The timestamp parser now checks input length before matching literal characters. A regression test verifies that is_timestamp rejects input missing a trailing format literal.

Changes

Timestamp literal validation

Layer / File(s)	Summary
Literal bounds check and regression test cpp/src/strings/convert/convert_datetime.cu, cpp/tests/strings/datetime_tests.cpp	The literal matching path checks the remaining input length before dereferencing the input pointer. A test covers an input missing a trailing literal.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to bfd09

This localized fix adds a length check to prevent invalid reads and includes a regression test; no actionable merge-blocking risk remains beyond normal checks and review.

Suggested reviewers: pointkernel

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% 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
Linked Issues check	✅ Passed	The code adds the required length guard and a regression test for issue #23660.
Out of Scope Changes check	✅ Passed	The implementation and test are limited to the linked out-of-bounds read in is_timestamp literal handling.
Title check	✅ Passed	The title clearly and concisely describes the added length check in cudf::strings::is_timestamp logic.
Description check	✅ Passed	The description directly explains the length check, the out-of-bounds read prevention, and the added test.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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