make activeSpan protected

[toddbeckman-wf]

This only limits the public API without affecting functionality.

Testing:

• CI passes

[aviary2-wf]

Security Insights

No security relevant content was detected by automated scans.

Action Items

• Review PR for security impact; comment "security review required" if needed or unsure
• Verify aviary.yaml coverage of security relevant code

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[codecov-io]

Codecov Report

Merging #122 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #122   +/-   ##
=======================================
  Coverage   94.18%   94.18%           
=======================================
  Files           4        4           
  Lines         413      413           
=======================================
  Hits          389      389           
  Misses         24       24

Impacted Files	Coverage Δ
lib/src/lifecycle_module.dart	96.5% <ø> (ø)	⬆️

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[greglittlefield-wf]

+10

[toddbeckman-wf]

@Workiva/release-management-p