fix: honor WithHTTPClient for WebSocket dial via coder/websocket DialOptions

[sruehl]

Fixes #246.

What changed

• httpconnection.go: when the caller's Doer is an *http.Client, populate DialOptions.HTTPClient before calling websocket.Dial so that coder/websocket uses the caller's transport for the WebSocket upgrade handshake.
• Updated WithHTTPClient godoc to reflect the new behaviour and document the *http.Client requirement for WebSocket propagation.
• httpserver_test.go: added a test that starts a httptest.NewTLSServer, connects a client with WithHTTPClient(testServer.Client()) and WithTransports(TransportWebSockets), and asserts a successful Invoke round-trip.

Background

Issue #136 removed the custom client from the WebSocket path (for nhooyr.io/websocket) because custom transports that do not return writable response bodies broke the dial. The fix was correct for that library. Since the migration to github.com/coder/websocket, DialOptions.HTTPClient is explicitly supported and its docs note the writable-body constraint (http.Transport satisfies this since Go 1.12).

The type assertion (httpConn.client.(*http.Client)) is required because httpConn.client is typed as Doer while DialOptions.HTTPClient is *http.Client. Callers passing a non-*http.Client Doer continue to fall back to http.DefaultClient for the WebSocket dial, as before.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.91%. Comparing base (df663fa) to head (d168939).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
+ Coverage   78.56%   78.91%   +0.34%     
==========================================
  Files          30       30              
  Lines        2431     2433       +2     
==========================================
+ Hits         1910     1920      +10     
+ Misses        386      380       -6     
+ Partials      135      133       -2     

Files with missing lines	Coverage Δ
httpconnection.go	52.17% <100.00%> (+7.04%)	⬆️

... and 2 files with indirect coverage changes

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[sruehl]

@philippseith ping