Improve integration test coverage and set threshold

[pdp2121]

Add more integration tests and set appropriate integration coverage threshold in CI (for reference, xrpl-py coverage threshold for integration test is at 70%).

This should also include CLI, async network clients (asynch/clients/**), sync wrappers around network calls (account/, ledger/, transaction/mod.rs), and the faucet client tests.

[satyakwok]

Submitted the infra half of this issue in #305 — wraps the existing integration test step in cargo llvm-cov, prints the per-run summary, and uploads the lcov.info as an artifact.

Left the --fail-under-* thresholds unset on purpose: the right numbers depend on what the suite actually covers today, the same way unit_test.yml was tuned to its measured baseline. Once a few PRs from this branch run and the percentages stabilise, the gate can be added in a follow-up.

The "add more integration tests" half of this issue (asynch::clients/**, account/, ledger/, transaction/mod.rs, faucet) is out of scope for this PR — that work needs the baseline first.