Only include context observer block when workgroup types available.

This allows real-time threads to join an audio workgroup on Apple platforms.

Previously, the Lock was not thread-safe when multiple threads were
locking/unlocking a single Lock instance simultaneously. This
isn't normally a problem when using the MessageManagerLock type, because
each MessageManagerLock contains its own private MessageManager::Lock,
and it's not possible for multiple threads to modify that private lock.

This change improves safety when using a MessageManager::Lock in the
manner of a CriticalSection or other JUCE lock type.

Status codes are already filtered in curlHeaderCallback, so there's no
need to remove them again in parseHttpHeaders. Request headers will
never include a status, so there's no need to remove the status in that
case.

Co-authored-by: LeonPS <leonps531@gmail.com>