test: cover SharedStream waiting waker replacement

[lance-gatefixer]

Summary

• add deterministic regression coverage for repeatedly polling the waiting half of a SharedStream
• protect the existing behavior that replaces a stale waiting waker

Root cause and repair

A consumer may be polled more than once while the other half owns polling of the inner stream. The original implementation asserted that no waiting waker was already stored, but repeated or spurious polls are valid and triggered the panic reported in #2391. Current main already contains the production correction from #3079, which replaces the stale waker; this PR adds the missing deterministic regression test that fails with the original assertion.

Validation

• cargo test -p lance-core
• cargo fmt --all
• cargo clippy --all --tests --benches -- -D warnings

Fixes #2391

[lance-gatekeeper]

✅ Gate recommendation: approve.

The deterministic regression test covers the valid repeated-poll path that previously triggered SharedStream’s stale-waker assertion, without changing production behavior.