chore: migrate from deprecated absl types to std equivalents

[scotthart]

fixes #16354

[gemini-code-assist]

Code Review

This pull request migrates various Abseil utilities (such as absl::variant, absl::visit, absl::holds_alternative, absl::get, absl::conjunction, absl::disjunction, and absl::void_t) to their standard C++17 library equivalents (std::variant, std::visit, etc.) across the codebase. The review feedback identifies a potential compilation failure in default_row_reader.cc due to mismatched return types, an unused Abseil header in list_objects_and_prefixes_reader.h, and a missed migration from absl::optional to std::optional in validate_metadata.cc.

[gemini-code-assist]

The return type of DefaultRowReader::Advance() (and potentially AdvanceOrFail()) still appears to be absl::variant (as seen in the hunk header). If AdvanceOrFail() returns absl::variant, using std::holds_alternative and std::get on it will fail to compile. Please ensure that the return types of both Advance() and AdvanceOrFail() are also migrated to std::variant in the corresponding header and source files to fully adhere to the style guide.

References

1. Use std::variant instead of absl::variant. ^(link)

[gemini-code-assist]

Since absl::visit has been migrated to std::visit in this file, the #include "absl/types/variant.h" on line 20 is likely no longer needed and should be removed to keep the includes clean.

References

1. Use std::variant instead of absl::variant. ^(link)

[gemini-code-assist]

On the next line, absl::optional<std::string> is still used as the return type for Get. Please migrate it to std::optional<std::string> to adhere to the repository style guide.

References

1. Use std::optional instead of absl::optional. ^(link)

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.26%. Comparing base (80d2a81) to head (1826c0b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...torage/internal/async/writer_connection_resumed.cc	80.00%	2 Missing ⚠️
generator/internal/http_option_utils.cc	80.00%	1 Missing ⚠️
...orage/internal/async/writer_connection_buffered.cc	92.30%	1 Missing ⚠️
...d/storage/internal/async/writer_connection_impl.cc	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16400      +/-   ##
==========================================
- Coverage   92.26%   92.26%   -0.01%     
==========================================
  Files        2246     2246              
  Lines      212121   212121              
==========================================
- Hits       195721   195705      -16     
- Misses      16400    16416      +16     

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