Fix serial fd leak in SerialConnection.connect() on timeout

[joeyleake]

Fixes #95

Problem

SerialConnection.connect() leaked 3 file descriptors (the serial port fd
plus two internal pipe fds) every time a connection attempt timed out.
In a reconnect loop against an unresponsive device, this exhausts the
~1024 fd limit and starts failing with "Too many open files".

Root cause

serial_asyncio.create_serial_connection() opens the fds immediately, but
the transport was only ever captured via self.transport, which is
assigned inside MCSerialClientProtocol.connection_made() — an async
callback. If asyncio.wait_for(self._connected_event.wait(), ...) timed
out before connection_made() ran, self.transport stayed None, so there
was no way to close the already-opened transport. It was simply
abandoned.

Fix

connect() now captures transport directly from create_serial_connection()'s
own return value instead of relying on self.transport being set. If the
subsequent wait_for() raises for any reason (timeout or otherwise), the
transport is closed before re-raising. Added a narrow guard
(if self.transport is transport: self.transport = None) for the case
where connection_made() fires right at the timeout boundary, so closing
this attempt's transport never clobbers self.transport if it's already
pointing at a different, unrelated connection. The success path
(logging + return self.port) is unchanged.

Testing

Added tests/unit/test_serial_connection.py::test_connect_closes_transport_on_timeout,
which patches create_serial_connection to return a transport whose
connection_made() never fires, and asserts transport.close() is called
before the TimeoutError propagates.

Full suite: 180 passed, 2 failed (test_send_cmd,
test_advert_path_preserves_embedded_zero_bytes) — both pre-existing on a
clean main checkout, unrelated to this change.