[BUG](worker): Detach fn-consumer batches

[tanujnay112]

Summary

Detach each FnConsumerManager::dispatch_batch span from the scheduled poll trace.

Why

A scheduled poll can run multiple batches concurrently via join_all, so keeping those spans under the poll conflates independent attached-function executions in one trace. Each batch is now its own trace root, while the scheduled-poll trace retains polling and GetWork visibility.

Impact

Function-batch traces can be inspected independently in Honeycomb. This changes trace topology only; it does not change function execution behavior or span volume.

Validation

• git diff --check
• cargo fmt --check
• cargo test -p worker fn_consumer --lib

[github-actions]

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