fix: avoid infinite retry in blue-green migration test

[nodece]

Motivation

The extensible-load-manager CI job timed out in TestBlueGreenMigrationTestSuite/TestTopicMigration/proxyConnection after 5 minutes.

From the stack and logs, the test was stuck waiting on WaitGroup while producer/consumer goroutines were still looping in retry paths. During migration, producer can enter terminal states (for example TopicTerminated or ProducerClosed), but the test retry loops had no terminal-exit logic, causing effectively unbounded retries and suite timeout.

Modifications

• Add terminal error handling in producer send retry loop:
  • if error is ErrTopicTerminated or ErrProducerClosed, fail fast instead of retrying forever.
• Add bounded retry windows for both producer and consumer loops (30 seconds per message stage).
• Add an error channel and stage-level wait helper around WaitGroup waits to fail early on goroutine errors.
• Add stage timeout protection while waiting for:
  • pre-unload send/receive synchronization
  • producer/consumer goroutine completion
• Keep per-iteration context cancellation immediate (cancel after each send/receive attempt).

These changes make the test deterministic under migration failures and prevent hanging until global test timeout.