fix(fluentd): parse retry queue length from yaml

[dajiaohuang]

Summary

• parse the Fluentd retry_queue_len YAML option
• cover the parsed value in the existing configuration test

This mirrors the G3-side support requested during review of bytedance/g3#1119.

Validation

• cargo fmt --all --check
• cargo test -p vey-fluentd --features yaml (cannot reach project tests in my Windows environment because OpenSSL development libraries are unavailable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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