fix: remove default case in waitWithContext to prevent busy-spin deadlock

[nodece]

Fixes #1455

Motivation

When the memory limit is exhausted, ReserveMemory() enters a loop that calls waitWithContext() to block until memory is released. However, waitWithContext() contained a default branch in its select statement that caused it to return immediately without ever blocking:

select {
case <-n:
    return true
case <-ctx.Done():
    return false
default:
    return true  // ← always fires, never waits
}

1. Consumer messages fill the shared memory limit via ForceReserveMemory
2. App goroutines processing those messages call producer.Send() (blocking mode)
3. ReserveMemory() spins at 100% CPU, never sleeping
4. Since all processing goroutines are stuck spinning, no messages are ever acked — consumer memory is never released
5. TryReserveMemory keeps failing → the spin loop never exits

This matches the reported symptoms exactly: apparent deadlock with very high CPU usage.

Modifications

• pulsar/internal/channel_cond.go: Remove the default branch from waitWithContext(). The function now properly blocks on the channel until either ReleaseMemory() triggers a broadcast (memory freed) or the context is cancelled. This converts the busy-spin into a correct condition-variable wait — goroutines sleep while memory is exhausted and wake up only when memory becomes available.

• pulsar/internal/channel_cond_test.go: Add TestChCondWithContextBlocks to explicitly verify that waitWithContext actually blocks and does not return via a default case, preventing regression.