Fix text output returned despite --output json in detached mode

[Copilot]

StartDetachedProcessAsync in Program.cs writes directly to Console.Out/Console.Error before the DI container and logging infrastructure are initialized, bypassing JsonConsoleFormatter entirely. The --output json flag was ignored.

• Added IsJsonOutputRequested helper that parses --output json from raw args (same pattern as ConfigCommand.RunValidateStandaloneAsync)
• Success output uses the same JSONL envelope as JsonConsoleFormatter: {"type":"result","data":{pid,apiUrl,logFile},"timestamp":"..."}
• Error paths emit JSONL log entries to stderr: {"type":"log","level":"error","message":"...","timestamp":"..."}
• All JSON output is single-line JSONL for consistent machine parsing across commands

With --output json (success):

{"type":"result","data":{"pid":6456,"apiUrl":"http://127.0.0.1:8897","logFile":"/Users/scope/.local/dev-proxy/logs/devproxy-6456-2026-03-05.log"},"timestamp":"2026-03-05T14:22:42.0000000Z"}

With --output json (error):

{"type":"log","level":"error","message":"Dev Proxy is already running (PID: 6456). Use 'devproxy stop' to stop it first.","timestamp":"2026-03-05T14:22:42.0000000Z"}

Without (unchanged):

Dev Proxy started in background.

  PID:       6456
  API URL:   http://127.0.0.1:8897
  Log file:  /Users/scope/.local/dev-proxy/logs/devproxy-6456-2026-03-05.log

Original prompt

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This section details on the original issue you should resolve

<issue_title>Bug: text output returned despite --output json</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.

Despite output being set to json the actual output is plain text.</issue_description>

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

• Fixes Bug: text output returned despite --output json #1587

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[Copilot]

Pull request overview

Fixes detached-mode startup output so devproxy --detach --output json returns JSON instead of always returning text, even before DI/logging is initialized.

Changes:

• Detects --output json from raw args in StartDetachedProcessAsync.
• Emits JSON on stdout for successful detached startup and JSON on stderr for error paths.
• Adds IsJsonOutputRequested helper and uses ProxyUtils.JsonSerializerOptions for serialization.

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[waldekmastykarz]

@copilot apply changes based on the comments in this thread

[Copilot]

@copilot apply changes based on the comments in this thread

Applied in a65e725. JSON output now uses the same JSONL envelope format as JsonConsoleFormatter:

• Success: {"type":"result","data":{"pid":6456,"apiUrl":"http://127.0.0.1:8897","logFile":"..."},"timestamp":"..."}
• Errors: {"type":"log","level":"error","message":"...","timestamp":"..."}

All output is single-line JSONL for consistent machine parsing across commands.