feat(websockets): add manual WebSockets.upgrade(f, stream) for mixed HTTP/WS servers

[quinnj]

Summary

HTTP.jl 1.x let you upgrade an in-flight server stream to a WebSocket by hand
(WebSockets.upgrade(f, http::Stream)), which is how a single HTTP.listen! /
HTTP.Router server could serve both ordinary HTTP routes and WebSocket routes.
The 2.0 rewrite only shipped the standalone WebSockets.listen! accept loop, so
this manual path went missing. This PR restores it.

HTTP.listen!("127.0.0.1", 8080) do stream
    if HTTP.WebSockets.isupgrade(stream.message)
        HTTP.WebSockets.upgrade(stream) do ws
            for msg in ws
                HTTP.WebSockets.send(ws, msg)
            end
        end
    else
        HTTP.setstatus(stream, 200)
        HTTP.startwrite(stream)
        write(stream, "ok")
    end
end

How it works

• Writes the 101 handshake straight to the connection, then takes the socket
  away from the HTTP/1 server loop using the existing write_closed /
  read_closed flags plus response.close, so the loop's post-handler
  teardown becomes a no-op and it stops reading further requests off what is now
  a WebSocket.
• Clears the per-request read deadline the server armed before invoking the
  handler, so a long-lived WebSocket isn't torn down mid-session (covered by a
  test that idles longer than read_timeout).
• Hands any bytes buffered past the handshake request to the WebSocket codec
  (mirrors the existing client handshake path).
• Rejects upgrades over HTTP/2 with a clear error.

Changes

• WebSockets.upgrade(f, stream) — new public API.
• _upgrade_response parameterized by subprotocols / check_origin; the
  Server method is now a thin wrapper over it (no behavior change — existing
  server origin / subprotocol / invalid-key tests still pass).
• Server Streams now retain their per-connection reader so upgrade can
  recover buffered bytes. Server body reads go through request_body, so
  stream.reader is otherwise unused on the server side.
• Migration guide section documenting upgrade.

Tests

New integration tests: mixed HTTP + WS on one listen! (text + binary echo),
the read-deadline-clearing case, and rejection of non-upgradeable streams.
Existing WebSocket client / server / integration suites and the HTTP/1 server
suite still pass locally.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 68.08511% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.36%. Comparing base (47fd097) to head (dc79fe1).

Files with missing lines	Patch %	Lines
src/http_websockets.jl	67.39%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1255      +/-   ##
==========================================
- Coverage   84.50%   84.36%   -0.14%     
==========================================
  Files          28       28              
  Lines       10648    10689      +41     
==========================================
+ Hits         8998     9018      +20     
- Misses       1650     1671      +21     

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