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 84.37%. Comparing base (83f4873) to head (560ccf7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1261      +/-   ##
==========================================
- Coverage   84.51%   84.37%   -0.14%     
==========================================
  Files          28       28              
  Lines       10690    10691       +1     
==========================================
- Hits         9035     9021      -14     
- Misses       1655     1670      +15     

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