out_http: Fix bug where post_all_requests() always returns a value less than 0

[nicehide]

When using body_key and headers_key in output:http, post_all_requests() is called in cb_http_flush(). The exit condition of the while(ret = flb_log_event_decoder_next()) loop is stored in 'ret' and returned as is. This leads to an error being recognized in cb_http_flush(), resulting in a "failed to post requests body" error, and input tasks accumulating in a pending state, causing continuous memory consumption growth. This has been fixed.

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Testing
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Documentation

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Backporting

• [N/A] Backport to latest stable release.

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