test(net/tx): add transaction fetcher benchmark harness

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Summary

• add a manager-level harness that drives transaction announcements through TransactionsManager
• cover retry and import behavior through deterministic mock peer sessions
• add disjoint and overlapping Criterion workloads with untimed equal-work assertions

This gives the legacy fetcher and fetcher2 the same correctness and profiling boundary without exposing either implementation or changing the production fetcher. The measured region excludes setup, teardown, and request-stat collection, and the workload fits the manager's default capacity so both implementations complete identical work.

Part of the incremental rollout tracked in #26410.

Validation

• cargo +nightly fmt --all -- --check
• cargo +nightly clippy -p reth-network --lib --benches --tests --all-features -- -D warnings
• cargo test -p reth-network --lib --features test-utils (184 passed)
• cargo bench -p reth-network --bench transaction_fetcher --features test-utils -- --test
• the same test and benchmark workloads pass unchanged on fetcher2 prototype 4970010438