Move ostruct gem out from test group for Ruby 3.5

[y9v]

Apparently ostruct gem is removed by bundle clean before the memory leak check.

What does this PR do?
This PR moves ostruct gem out from the test group.

Motivation:
CI failing:
https://github.com/DataDog/dd-trace-rb/actions/runs/12713392872/job/35441183658?pr=4236

Change log entry
None.

Additional Notes:
None.

How to test the change?
CI is enough

[datadog-datadog-prod-us1]

Datadog Report

Branch report: add-ostruct-gem-to-ruby-3-5-gemfile
Commit report: ecf6a2b
Test service: dd-trace-rb

✅ 0 Failed, 22186 Passed, 1476 Skipped, 5m 23.82s Total Time

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.71%. Comparing base (5235204) to head (ecf6a2b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4287      +/-   ##
==========================================
- Coverage   97.72%   97.71%   -0.02%     
==========================================
  Files        1354     1354              
  Lines       82410    82410              
  Branches     4213     4213              
==========================================
- Hits        80538    80525      -13     
- Misses       1872     1885      +13     

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-14 19:19:14

Comparing candidate commit ecf6a2b in PR branch add-ostruct-gem-to-ruby-3-5-gemfile with baseline commit 5235204 in branch master.

Found 3 performance improvements and 0 performance regressions! Performance is the same for 28 metrics, 2 unstable metrics.

scenario:tracing - 100 span trace - no writer

• 🟩 throughput [+17.044op/s; +17.884op/s] or [+5.157%; +5.411%]

scenario:tracing - Propagation - Datadog

• 🟩 throughput [+3619.499op/s; +3695.877op/s] or [+12.139%; +12.395%]

scenario:tracing - Tracing.log_correlation

• 🟩 throughput [+7183.597op/s; +7603.163op/s] or [+6.559%; +6.942%]

[ivoanjo]

Thanks for fixing this -- I think an alternative could be to tweak the bundle install that we use to install the test group, but I think this option works well too. (Especially given nothing else is using Ruby 3.5 on our tests yet. Seems so far away, 3.5 haha ... the future)