[FEATURE]: Improve UX for Port Conflicts - Show Clear Error Instead of Freezing #19272

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Summary
When launching multiple OpenCode instances with a fixed server port configured, the second instance can freeze without any feedback. This creates confusion and makes it appear as though the tool is unresponsive or single-session only.

Problem
If a user configures a fixed port (e.g. "server": { "port": 4096}) and then starts a second OpenCode instance:

• The port is already in use by the first instance
• The second instance attempts to bind to the same port
• Instead of failing gracefully, the process appears to hang/freeze with no message

Important:
Even when running with maximum verbosity:

opencode --log-level DEBUG

there is no output, error, or diagnostic message indicating that a port conflict has occurred.

This results in:

• Poor developer experience (unclear what went wrong)
• Misleading behavior (looks like OpenCode does not support multiple sessions)
• Debugging difficulty (even DEBUG logs provide no signal)
• Time wasted investigating unrelated causes

Expected Behavior
When the configured port is already in use, OpenCode should:

1. Fail fast instead of hanging
2. Display a clear, actionable error message, e.g.:

Error: Port 4096 is already in use.

Another OpenCode instance may already be running.
You can:
- Stop the existing instance
- Use a different port (e.g. --port 4097)
- Remove the fixed port from your config to allow auto-assignment

3. Ensure the error is visible at all log levels, including default and DEBUG

Proposed Enhancements (Optional)

• Add a --port auto fallback mode:

  • If the configured port is busy, automatically pick the next available port

• Emit explicit logs during server startup (e.g. “binding to port…”)

• Detect and display the PID/process holding the port (if feasible)

• Add a pre-flight port availability check before attempting to start

Impact
This change would:

• Prevent confusion for new users
• Improve reliability perception
• Make DEBUG logging actually useful for troubleshooting
• Reduce support/debugging overhead
• Align behavior with standard CLI/server UX expectations

Environment

• OpenCode version: 1.3.0
• OS: Linux
• Config:

{
  "server": {
    "port": 4096
  }
}

Repro Steps

1. Configure a fixed port in config.json
2. Start OpenCode in one folder
3. Start OpenCode in another folder
4. Observe second instance freezing with no output
5. Retry with --log-level DEBUG → still no indication of port conflict