in_emitter: in_forward: Ensure ring buffer on threaded mode in emitter [Backport to 4.0]

[cosmo0920]

backporting of #10950.

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✨ Finishing touches

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🧪 Generate unit tests

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• Commit unit tests in branch cosmo0920-ensure-ring-buffer-on-threaded-mode-in-emitter-4.0

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[niedbalski]

🔄 Backported to FluentDo Agent

This change has been backported to FluentDo Agent.

• FluentDo PR: feat: sync upstream Fluent Bit from v4.0.11 to upstream/4.0 telemetryforge/agent#18
• FluentDo PR Title: feat: sync upstream Fluent Bit from v4.0.11 to upstream/4.0
• Upstream Commit: 269b951

You can view the test results and integration status in the FluentDo Agent PR.

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