Fix log db batch flush flake

[dylan-hurd-oai]

Why

The log DB writer batches tracing events before inserting them into SQLite, but tokio::time::interval produces an immediate first tick. That meant the inserter could flush the first accepted log entry before batch_size was reached, making configured_batch_size_flushes_without_explicit_flush timing-sensitive in CI.

What Changed

• Consume the interval's startup tick before entering the inserter loop, so interval flushing starts after the configured delay.
• Remove the test's startup sleep, which was masking the race instead of proving the batch-size behavior.

Validation

• cargo test -p codex-state
• cargo test -p codex-state configured_batch_size_flushes_without_explicit_flush passed 3 consecutive focused runs
• PR checks passed across rust-ci, Bazel, ci, sdk, cargo-deny, Codespell, blob-size policy, and CLA