test(amqp091): join connectionWatcher goroutines in test teardown

[miotte]

Fixes #108

Stacked on #102 (miotte-pr7), which makes clientDisconnect atomic — this branch's teardown calls Disconnect in ~45 tests, and that would otherwise reintroduce the #101 race. A fully clean -race run of the amqp091 reconnect tests also needs #104 (atomic bd.state). Verified on a branch combining the whole stack (see Test plan). Once #102 merges, retarget this PR to main.

Summary

• Add a sync.WaitGroup to BrokerDetails so the connectionWatcher goroutine started by Connect is joinable (the only production change).
• Add a stopWatcher test helper that disconnects a client and waits for its watcher to exit, and defer it in every Connect-based test so a watcher can no longer outlive its test and either race the package-level NewAmqpConn091 swap (read in connect()) or panic calling IsClosed() on a stale mock from its 30s fallback branch.
• The helper skips the disconnect when the test already triggered one (guarding on clientDisconnect.Load()), so it can't block on a second shutdownChan send.

Test plan

Verified on a branch combining this change with the full race-fix stack (#82, #84, #97, #98, #102, #104):

• go test -race -run "Test_Retry|Test_WaitForConnect|Test_connect_connecting_" -count=5 ./internal/provider/connectors/amqp091/... — 12/12 runs pass (was ~75% before this fix).
• go test -race -run "Test_Retry|Test_WaitForConnect|Test_connect_connecting_|Test_StreamRetry|Test_Disconnect$|Test_SubscribeStream" -count=5 ./internal/provider/connectors/amqp091/... — 6/6 pass (was 0/8 + a 10-minute hang before).
• go test -race -count=5 ./internal/provider/connectors/amqp091/... — 3/3 pass.
• go test ./... -race — passes across all packages.
• go vet ./internal/provider/connectors/amqp091/... and gofmt clean.