Implement WebSocket proxying

[fifthsegment]

Replaces the WebSocket stub (which returned an error) with a proper TCP tunnel implementation.

gatesentryproxy/websocket.go```: Implement `HandleWebsocketConnection` to:
1. Hijack the client HTTP connection
2. Dial the upstream server via TCP
3. Forward the HTTP upgrade request in origin-form
4. Bridge bidirectional data between client and upstream (raw TCP tunnel)

The proxy already detected WebSocket upgrades at `proxy.go:383` and routed them to `HandleWebsocketConnection` — the function just returned an error. This change implements the actual proxying.

- **ws://** connections: handled by this implementation
- **wss://** connections: already worked via the existing CONNECT tunnel

Builds clean: `go build ./...` passes.

[github-actions]

🚀 PR Build Artifacts

Test binaries have been built for this PR (commit a169ff7).

Available Binaries:

• gatesentry (0.00 MB)
• gatesentry-linux-amd64 (18.82 MB)
• gatesentry-linux-arm64 (17.89 MB)
• gatesentry-macos-amd64 (18.66 MB)
• gatesentry-macos-arm64 (17.98 MB)
• gatesentry-windows-amd64.exe (16.60 MB)

Download Instructions:

1. Via GitHub Actions UI:

   • Go to the Actions tab
   • Scroll down to "Artifacts" section
   • Download gatesentry-pr-150

2. Via GitHub CLI:

   gh run download 25992177795 -n gatesentry-pr-150 -R fifthsegment/Gatesentry

Quick Test (Linux/macOS):

chmod +x gatesentry-*
./gatesentry-linux-amd64

Quick Test (Windows):

.\gatesentry-windows-amd64.exe

---

📝 Binaries are kept for 30 days and automatically deleted afterwards.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 19.19%. Comparing base (8074ddc) to head (a169ff7).
⚠️ Report is 3 commits behind head on master.
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #150   +/-   ##
=======================================
  Coverage   19.19%   19.19%           
=======================================
  Files          70       70           
  Lines        4152     4152           
=======================================
  Hits          797      797           
  Misses       3310     3310           
  Partials       45       45           

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