chore(bigquery): migrate to std::optional

[colinmoy]

Replacing explicit usages of absl::optional with std::optional in the BigQuery Client library as part of broader modernization efforts

[gemini-code-assist]

Code Review

This pull request replaces the use of absl::optional with std::optional across several files in the BigQuery v2 minimal internal library, aligning with C++17 standards. The reviewer suggests returning std::optional<std::chrono::system_clock::time_point> by value instead of by const& in ListJobsRequest because it is a small, trivially copyable type, which avoids pointer indirection and potential lifetime issues.

[gemini-code-assist]

Since std::chrono::system_clock::time_point is a small, trivially copyable type, std::optional<std::chrono::system_clock::time_point> is also small and cheap to copy. Returning it by value instead of by const& is more idiomatic in C++, avoids pointer indirection overhead, and prevents potential lifetime issues (dangling references) for callers.

  std::optional<std::chrono::system_clock::time_point>
  min_creation_time() const {
    return min_creation_time_;
  }
  std::optional<std::chrono::system_clock::time_point>
  max_creation_time() const {
    return max_creation_time_;
  }

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.24%. Comparing base (e0b2850) to head (6b36030).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16210      +/-   ##
==========================================
- Coverage   92.24%   92.24%   -0.01%     
==========================================
  Files        2265     2265              
  Lines      210126   210126              
==========================================
- Hits       193839   193828      -11     
- Misses      16287    16298      +11     

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