Move CI env helpers to test file

[osterman]

Summary

• restrict CI env helpers to tests only
• remove ci_env.go from testutils package
• keep package buildable with minimal testutils.go

Testing

• make lint (fails: can't load config)
• make testacc-cover (interrupted)
• go test ./internal/testutils -run TestPreserveRestoreCIEnvVars (pass)
• go test ./... (interrupted)

---

https://chatgpt.com/codex/tasks/task_b_6859b2092c788332825df3e63fbe4266

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 82.60870% with 4 lines in your changes missing coverage. Please review.

Please upload report for BASE (DEV-2965/implement-atmos-telemetry@c0ae6cf). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
internal/testutils/ci_env.go	82.60%	3 Missing and 1 partial :warning:

Additional details and impacted files

@@                          Coverage Diff                          @@
##             DEV-2965/implement-atmos-telemetry    #1333   +/-   ##
=====================================================================
  Coverage                                      ?   51.07%           
=====================================================================
  Files                                         ?      244           
  Lines                                         ?    26113           
  Branches                                      ?        0           
=====================================================================
  Hits                                          ?    13338           
  Misses                                        ?    11127           
  Partials                                      ?     1648           

Flag	Coverage Δ
unittests	51.07% <82.60%> (?)

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[mergify[bot]]

💥 This pull request now has conflicts. Could you fix it @osterman? 🙏