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 91.89189% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.24%. Comparing base (e0b2850) to head (22d6ff4).
⚠️ Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
generator/internal/descriptor_utils.cc	75.00%	1 Missing ⚠️
generator/internal/metadata_decorator_generator.cc	0.00%	1 Missing ⚠️
...ator/internal/metadata_decorator_rest_generator.cc	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16230      +/-   ##
==========================================
- Coverage   92.24%   92.24%   -0.01%     
==========================================
  Files        2265     2265              
  Lines      210126   210126              
==========================================
- Hits       193839   193826      -13     
- Misses      16287    16300      +13     

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