Print proxy URL when starting Dev Proxy in detach mode

[Copilot]

• Analyze merge conflicts between branch and main
• Resolve ProxyEngine.cs conflicts (take main's SaveInstanceStateAsync approach + imports)
• Resolve Program.cs conflicts (use LoadStateByPidAsync from main, keep Proxy URL output and port-ready waiting)
• Update UpdateStateWithApiUrlAsync in DevProxyCommand.cs to use per-PID state API
• Build and verify (0 warnings, 0 errors)
• Code review and security scan (0 alerts)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Print port number when starting Dev Proxy in detach mode</issue_title>
<issue_description>When you start Dev Proxy using devproxy --port 0 --output json --detach it shows the following output:

Dev Proxy started in background.

  PID:       6456
  API URL:   http://127.0.0.1:8897
  Log file:  C:\Users\scope\AppData\Local\dev-proxy\logs\devproxy-6456-2026-03-05-14-22-42.log

Use 'devproxy status' to check status.
Use 'devproxy logs' to view logs.
Use 'devproxy stop' to stop.

--port 0 means that a random port should be assigned to Dev Proxy. The output however doesn't show which port was assigned making it impossible to use Dev Proxy.

Between PID and API URL print the actual proxy URL and assigned port (even when explicitly specified through args)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Print port number when starting Dev Proxy in detach mode #1590

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[Copilot]

Pull request overview

Improves detached-mode startup output so callers can discover the actual proxy address when using OS-assigned ports (--port 0), by persisting the resolved proxy port and printing a Proxy URL in the parent process.

Changes:

• Update ProxyEngine to set the runtime port after the proxy endpoint is started, and (when running as the internal daemon) re-save the detached state file with the resolved port.
• Update detached-mode startup polling to wait for state.Port > 0 and print a new Proxy URL: line in the console output.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
DevProxy/Proxy/ProxyEngine.cs	After starting the proxy endpoint, updates the effective port and re-saves detached state with the resolved port for --port 0.
DevProxy/Program.cs	In detached startup output, waits for a non-zero port and prints the resolved Proxy URL alongside PID/API URL/log file.

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[garrytrinder]

Bug: API URL shows port 0 when using --api-port 0

While the proxy port resolution works correctly (the main goal of this PR), the same issue exists for the API port when --api-port 0 is used.

Steps to reproduce

devproxy --detach --as-system-proxy false --port 0 --api-port 0

Expected output

Dev Proxy started in background.

  PID:       4588
  Proxy URL: http://127.0.0.1:63860
  API URL:   http://127.0.0.1:64123

Actual output

Dev Proxy started in background.

  PID:       4588
  Proxy URL: http://127.0.0.1:63860
  API URL:   http://127.0.0.1:0

The API URL retains port 0 because ProxyInstanceState is saved in Program.cs before Kestrel binds to the actual port. The proxy port fix in ProxyEngine.ExecuteAsync updates _config.Port from _explicitEndPoint.Port after binding, but no equivalent resolution happens for the Kestrel API port.

Impact

• devproxy stop fails with Can't assign requested address (127.0.0.1:0) — must use --force
• devproxy status shows the wrong API URL
• Any tooling relying on the JSON output to discover the API endpoint gets an unusable URL

Suggestion

Read the actual Kestrel-bound port from IServerAddressesFeature after startup and update the state file, similar to how the proxy port is resolved from _explicitEndPoint.Port. PR #1589 already does this for the API port via IServer injection in ProxyEngine — worth aligning with that approach.

Not a regression (this existed before this PR), but since this PR specifically improves the --port 0 experience, it would be good to handle the API port too for a complete fix.

[waldekmastykarz]

With

devproxy --detach --as-system-proxy false --port 0 --api-port 0

you're likely testing the globally installed version rather than the one built from PR. This caught me too.

[garrytrinder]

Re-tested using the branch build (dotnet run --project DevProxy, not the globally installed devproxy):

dotnet run --project DevProxy -- --detach --as-system-proxy false --port 0 --api-port 0

Output (branch build, confirmed by Proxy URL line which only exists on this branch):

Dev Proxy started in background.

  PID:       22165
  Proxy URL: http://127.0.0.1:58160
  API URL:   http://127.0.0.1:0
  Log file:  /Users/garrytrinder/Library/Application Support/dev-proxy/logs/devproxy-22165-2026-03-11-09-24-44.log

Proxy URL correctly resolves the OS-assigned port (58160). API URL still shows port 0.

dotnet run --project DevProxy -- stop fails with:

Failed to connect to Dev Proxy API: Can't assign requested address (127.0.0.1:0)
Use --force to forcefully terminate the process.

Bug confirmed on branch build.

[garrytrinder]

Re-tested after the latest commits. All scenarios pass:

• --port 0 --api-port 0: Both Proxy URL and API URL now resolve to real ports ✅
• --output json: Valid JSONL with correct proxyUrl and apiUrl ✅
• Explicit ports (--port 9080 --api-port 9081): Works correctly ✅
• stop command: Works in all cases ✅

The API URL port 0 bug is fixed. Nice work.

[garrytrinder]

@copilot resolve the merge conflicts