test(ads-client): assert shutdown is idempotent

[Almaju]

Adds a regression test covering repeated shutdown() calls on the ads client.

MozAdsClient::shutdown takes &self and goes through a lock, so a double shutdown is reachable from the foreign side. Both pieces of state it tears down are already Option::take() — MozAdsTelemetryWrapper::shutdown (ffi/telemetry.rs) and MARSTransport::shutdown_db (mars/transport.rs) — so the behaviour is correct today; this test pins it down.

The test builds the client with a real HttpCache, since that's where a regression would bite: HttpCache::shutdown_db(self) consumes and calls conn.close(), so replacing the take() with a borrow would make the second close error.

Pull Request checklist

• Breaking changes: no API changes; test-only.
• Quality: cargo test -p ads-client passes (96 passed, 1 ignored, 0 failed); cargo fmt --check and cargo clippy -p ads-client --all-targets are clean.
• Tests: this PR is the test. Mutation-checked it: temporarily making MARSTransport::shutdown_db return Err(InvalidQuery) on an already-shut-down cache makes it fail; restored and re-verified green.
• Changelog: not needed — no consumer-visible change.
• Dependencies: no new dependencies.

[thesuzerain]

Looks good to me