Fix Error When shutdown_agent called from Within Harvest Thread

[TimPansino]

Overview

Add a guard to prevent crashes when calling shutdown_agent from within the harvest thread.

Related Github Issue

Closes #1548

Testing

Not sure if this is possible to test, but seeing as it's a minuscule change that just adds a guard I think it's fine as is.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	7		0	0	0.93s
✅ MARKDOWN	markdownlint	7	0	0	0	1.31s
✅ PYTHON	ruff	948	0	0	0	1.03s
✅ PYTHON	ruff-format	948	0	0	0	0.35s
✅ YAML	prettier	15	0	0	0	1.55s
✅ YAML	v8r	15		0	0	5.28s
✅ YAML	yamllint	15		0	0	0.69s

See detailed reports in MegaLinter artifacts

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.75%. Comparing base (59633ac) to head (7a546f9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
newrelic/core/agent.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1552      +/-   ##
==========================================
- Coverage   81.76%   81.75%   -0.02%     
==========================================
  Files         207      207              
  Lines       23943    23946       +3     
  Branches     3796     3796              
==========================================
  Hits        19576    19576              
- Misses       3101     3103       +2     
- Partials     1266     1267       +1     

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