Rename constant conflicting with built-in type: Base64

[marcotc]

What does this PR do?
Renames Datadog::Core::Utils::Base64 to Datadog::Core::Utils::Base64Codec.

Motivation:
Datadog::Core::Utils::Base64 collides with Ruby's standard Base64 constant, when referenced through nested constant lookup.
This can cause you to reference Base64, normally wanting the std Base64 module, but end up with Datadog::Core::Utils::Base64.

Change log entry
None. Internal.

Additional Notes:
Added coverage for (the newly renamed) Base64Codec.

How to test the change?
bundle exec rspec spec/datadog/core/utils/base64_codec_spec.rb

[datadog-datadog-prod-us1]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 95.00%
• Overall Coverage: 97.09% (-0.00%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: d2c5694 | Docs | Datadog PR Page | Give us feedback!}

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-05-21 23:15:03

Comparing candidate commit d2c5694 in PR branch rename-base64-constant with baseline commit 3d0ced6 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 45 metrics, 1 unstable metrics.

Explanation

This is an A/B test comparing a candidate commit's performance against that of a baseline commit. Performance changes are noted in the tables below as:

• 🟩 = significantly better candidate vs. baseline
• 🟥 = significantly worse candidate vs. baseline

We compute a confidence interval (CI) over the relative difference of means between metrics from the candidate and baseline commits, considering the baseline as the reference.

If the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD), the change is considered significant.

Feel free to reach out to #apm-benchmarking-platform on Slack if you have any questions.

More details about the CI and significant changes

You can imagine this CI as a range of values that is likely to contain the true difference of means between the candidate and baseline commits.

CIs of the difference of means are often centered around 0%, because often changes are not that big:

---------------------------------(------|---^--------)-------------------------------->
                              -0.6%    0%  0.3%     +1.2%
                                 |          |        |
         lower bound of the CI --'          |        |
sample mean (center of the CI) -------------'        |
         upper bound of the CI ----------------------'

As described above, a change is considered significant if the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD).

For instance, for an execution time metric, this confidence interval indicates a significantly worse performance:

----------------------------------------|---------|---(---------^---------)---------->
                                       0%        1%  1.3%      2.2%      3.1%
                                                  |   |         |         |
       significant impact threshold --------------'   |         |         |
                      lower bound of CI --------------'         |         |
       sample mean (center of the CI) --------------------------'         |
                      upper bound of CI ----------------------------------'