feat(sidekick/rust): Improve generated samples format.

[amanda-tarafa]

This approach balances vertical vs. horizontal sample size. And we we don't need the newline-in-comment trick to keep the templates readable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.93%. Comparing base (d54fc6a) to head (c8a32e3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4067   +/-   ##
=======================================
  Coverage   81.93%   81.93%           
=======================================
  Files          78       78           
  Lines        6516     6516           
=======================================
  Hits         5339     5339           
  Misses        832      832           
  Partials      345      345           

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[gemini-code-assist]

Code Review

This pull request improves the formatting of generated Rust code samples, making them more readable by using a multi-line builder pattern. The changes are generally good, but I've found a critical issue where a leftover template tag will cause a syntax error in the generated code. I've also included a suggestion to improve consistency in the new formatting style.

[amanda-tarafa]

Ready for review, generation results can be seen on the second commit of googleapis/google-cloud-rust#4678.

@coryan I'm happy to further tweak this or try with cargo fmt, but I think this looks good.