feat(generator/rust): skip duplicate enum values

[coryan]

After converting the enum value names to Rust style we may find some
duplicates. With this change the generator can skip the dups. In all
practical cases the dups are harmless, they differ in case but have the
name numeric value. We warn if this assumption fails.

part of the work for #829

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.19%. Comparing base (58727a4) to head (ff9ed35).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1538   +/-   ##
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  Coverage   95.19%   95.19%           
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  Files          39       39           
  Lines        1603     1603           
=======================================
  Hits         1526     1526           
  Misses         77       77           

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[dbolduc]

This gave me pause. Maybe the generator should panic so we can go yell at the service team?

If we do receive the value 3, I guess we will just treat it as an UNKNOWN_VALUE:3 (or whatever). Which should be fine.

[coryan]

This gave me pause. Maybe the generator should panic so we can go yell at the service team?

I thought about that, but then we would stuck: sidekick update fails for all services, not just the offending one. We would need to hack something.

If we do receive the value 3, I guess we will just treat it as an UNKNOWN_VALUE:3 (or whatever). Which should be fine.

Exactly. It is very unlikely to happen (https://google.aip.dev/126 says enum values must be SCREAMING_SNAKE_CASE, the deviations are service bugs). And if it does happen we still can handle the incoming enums.