chore(bazel): add rules_cc loads to showcase and scaffold templates

[colinmoy]

Bazel 9 disables external rule autoloading (--incompatible_autoload_externally=), requiring explicit @rules_cc//cc:defs.bzl load() statements in generated BUILD.bazel files (ci/showcase/BUILD.bazel.in and GenerateQuickstartBuild in generator/internal/scaffold_generator.cc).

Part of b/563083065

[gemini-code-assist]

Code Review

This pull request updates Bazel build files and generator templates to explicitly load C++ rules (cc_library, cc_test, and cc_binary) from @rules_cc//cc:defs.bzl. Specifically, it updates ci/showcase/BUILD.bazel.in, the quickstart build template in generator/internal/scaffold_generator.cc, and the corresponding unit test in generator/internal/scaffold_generator_test.cc. I have no feedback to provide as the changes are correct and follow best practices.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.34%. Comparing base (d932dd6) to head (7184f42).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16532      +/-   ##
==========================================
- Coverage   92.35%   92.34%   -0.01%     
==========================================
  Files        2262     2262              
  Lines      217027   217029       +2     
==========================================
- Hits       200425   200420       -5     
- Misses      16602    16609       +7     

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