Add regex-flags member variable to internal libcudf reprog class

[davidwendt]

Description

Adds new member variable to hold the regex-flags specified by the regex-program interface so that it may be referenced more easily later for fast-path checking. This is some common changes needed in #22178 and #21936 and will help reduce the size of the changes for both and hopefully reduce confusion on the changes needed there.
Cleanup of related files is included as well.

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]

📝 Walkthrough

Walkthrough

The PR updates regex program declarations and implementation so reprog stores flags during construction and uses them in debug printing, and it adjusts count.cu’s anonymous-namespace scope and copyright header.

Changes

Regex program flags

Layer / File(s)	Summary
Header declarations cpp/include/cudf/strings/regex/regex_program.hpp, cpp/src/strings/regex/regcomp.h	The regex_program destructor moves into the public section, and reprog declares _flags, a regex_flags constructor, and print() const in the debug section.
Construction and print implementation cpp/src/strings/regex/regcomp.cpp	reprog::create_from constructs with flags, the new reprog(regex_flags) definition initializes _flags, and debug reprog::print() reads _flags directly.

Search count namespace cleanup

Layer / File(s)	Summary
Namespace boundary and header text cpp/src/strings/search/count.cu	The copyright text changes, the anonymous namespace closes after counter_fn, and the later closing brace after detail::count is removed.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested reviewers

• vyasr
• karthikeyann
• bdice

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% 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 accurately summarizes the main change: adding a regex-flags member to the internal reprog class.
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.
Description check	✅ Passed	The description matches the changeset: it adds regex-flags storage for regex-program and mentions related cleanup.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[vyasr]

/merge