Clean up some handling for decoder record and templates

[jrmolin]

Proposed commit message

filebeat: refactor NetFlow decoder record and template handling

This refactors some internal logic in the NetFlow v9 decoder to improve the robustness of record and template processing.

Checklist

• [x] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• [ ] I have added an entry in ./changelog/fragments using the changelog tool.

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[github-actions[bot]]

:robot: GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[vinit-chauhan]

LGTM.

[elasticmachine]

Pinging @elastic/integration-experience (Team:Integration-Experience)

[github-actions[bot]]

@mergifyio backport 8.19 9.1 9.2 9.3

[mergify[bot]]

backport 8.19 9.1 9.2 9.3

✅ Backports have been created

• #48179 [8.19](backport #48057) Clean up some handling for decoder record and templates has been created for branch 8.19
• #48180 [9.1](backport #48057) Clean up some handling for decoder record and templates has been created for branch 9.1
• #48181 [9.2](backport #48057) Clean up some handling for decoder record and templates has been created for branch 9.2
• #48182 [9.3](backport #48057) Clean up some handling for decoder record and templates has been created for branch 9.3