fix: fix flaky TestAsyncTransport_FlushWithContext

[giortzisg]

Description

Fix flaky TestAsyncTransport_FlushWithContext/timeout test.

The test was racy because it relied on timing to trigger a context cancellation:

• A context with a 1ns timeout was created, followed by a 10ms sleep to "ensure" expiry
• No envelope was sent, so the queue was empty
• When FlushWithContext was called, the worker could receive the flush request and drain the empty queue instantly, closing flushResponse, before the select evaluated ctx.Done()
• select would randomly pick between two true statements

The fix sends an envelope and makes the test server block indefinitely, so the flush can never complete within the context deadline.

Issues

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Semver Impact of This PR

🟢 Patch (bug fixes)

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