Run the UDS handshake once, via connect(), not also inside _connect()

[simpleqt]

Problem

valkey/asyncio/connection.py's UnixDomainSocketConnection._connect() ends with its own await self.on_connect(), but AbstractConnection.connect() also runs the handshake (on_connect, or the caller's valkey_connect_func) right after _connect() returns:

    async def _connect(self):
        async with async_timeout(self.socket_connect_timeout):
            reader, writer = await asyncio.open_unix_connection(path=self.path)
        self._reader = reader
        self._writer = writer
        await self.on_connect()      # <- connect() runs the handshake again

Every asyncio UDS connection therefore runs the default handshake twice (verified on master f3a73ec against a fake AF_UNIX RESP server that records commands):

TCP: 2 handshake commands -> [CLIENT SETINFO LIB-NAME, CLIENT SETINFO LIB-VER]
UDS: 4 handshake commands -> [the same two, twice]

Consequences:

• with password/username, AUTH credentials are sent twice per connection (and per reconnect), doubling credential traffic and round trips;
• a caller-supplied valkey_connect_func, which on TCP replaces the default handshake entirely, cannot do so on UDS — the default handshake still runs first:

TCP+custom: 1 command -> [CUSTOMINIT]
UDS+custom: 3 commands -> [CLIENT SETINFO LIB-NAME, CLIENT SETINFO LIB-VER, CUSTOMINIT]

Changes

_connect() now only opens the socket; the handshake is left to connect(), matching the TCP and SSL paths.

Testing

Two server-free tests in tests/test_asyncio/test_connection.py spin up a minimal AF_UNIX RESP server and count the handshake commands:

• on unpatched master: both fail (4 handshake commands; default handshake leaks in with valkey_connect_func) — red
• with this patch: both pass (2 commands; custom func replaces the handshake) — green

$ pytest tests/test_asyncio/test_connection.py -q -k handshake
2 passed

(flake8 / black --target-version py37 / isort clean on the touched files)

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