chore(generator): migrate to std::optional

[colinmoy]

Replacing explicit usages of absl::optional with std::optional in the internal code generator toolchain (generator/) as part of broader modernization efforts

[gemini-code-assist]

Code Review

This pull request migrates the usage of absl::optional and absl::nullopt to C++17's std::optional and std::nullopt across various generator, internal, and integration test files. I have no feedback to provide.

[codecov]

Codecov Report

❌ Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.26%. Comparing base (ce70c7d) to head (eb5abbf).

Files with missing lines	Patch %	Lines
generator/internal/descriptor_utils.cc	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16230      +/-   ##
==========================================
- Coverage   92.26%   92.26%   -0.01%     
==========================================
  Files        2214     2214              
  Lines      206307   206307              
==========================================
- Hits       190355   190352       -3     
- Misses      15952    15955       +3     

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