Fix C++ backend conditional optimization: formatting and golden file regeneration

[Copilot]

PR google#234 to google/emboss failed CI due to Black formatting violations and outdated golden test files after introducing the two-pass subexpression caching optimization for conditional fields.

Changes

• Python formatting: Applied Black 24.8.0 to run_one_golden_test.py and header_generator.py
• Golden file regeneration: Updated all 33 golden header files in testdata/golden_cpp/ to reflect the new Ok() method generation pattern

The new code generation wraps field existence checks in scoped blocks with cached subexpressions:

// Before
if (!has_x().Known()) return false;
if (has_x().ValueOrDefault() && !x().Ok()) return false;

// After
{
  const auto emboss_reserved_local_field_present = ${existence_condition};
  if (!emboss_reserved_local_field_present.Known()) return false;
  if (emboss_reserved_local_field_present.ValueOrDefault() && !x().Ok()) return false;
}

Special handling applied for no_enum_traits.emb.h (requires --no-cc-enum-traits flag).

Warning

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• releases.bazel.build
  • Triggering command: /usr/local/lib/node_modules/@bazel/bazelisk/bazelisk-linux_amd64 /usr/local/lib/node_modules/@bazel/bazelisk/bazelisk-linux_amd64 build //compiler/back_end/cpp:emboss_codegen_cpp //compiler/front_end:emboss_front_end (dns block)
• www.googleapis.com
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Original prompt

Fix my fork so that all targets in all BUILD files build correctly and all tests pass. Fix the code so that the presummit tests in pending pull request I submitted to google/emboss pass.

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