feat(websockets): add isready(ws) to poll for buffered messages

[quinnj]

Closes #1169.

Adds Base.isready(ws::WebSocket), which returns true when a complete message is buffered and ready to read with receive(ws) without blocking — a public, non-blocking way to check for incoming WebSocket messages. Previously this was only reachable by poking the internal read channel.

HTTP.WebSockets.open(url) do ws
    if isready(ws)
        msg = HTTP.WebSockets.receive(ws)   # guaranteed not to block
    end
end

isready semantics match Channel: false doesn't imply the connection is closed (more may arrive), and messages buffered before close stay isready and are returned by receive before it throws. Includes a docstring + integration test.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.36%. Comparing base (f9d62af) to head (9646ba9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1261   +/-   ##
=======================================
  Coverage   84.36%   84.36%           
=======================================
  Files          28       28           
  Lines       10689    10690    +1     
=======================================
+ Hits         9018     9019    +1     
  Misses       1671     1671           

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