feat: add --port flag to openhands serve

[singhharsh1708]

Why

openhands serve hardcodes host port 3000 for the Docker container (-p 3000:3000), with no way to override it. Since 3000 is also the default dev-server port for many frontend frameworks (Next.js, Vite, CRA), this creates a port conflict for developers who want to run OpenHands alongside their own project.

Summary

• Add --port <PORT> option to openhands serve, default 3000
• Map host --port to the container's fixed port 3000 (-p <PORT>:3000)
• Update the printed "server will be available at" URL to reflect the chosen port

Issue Number

Closes #779

How to Test

openhands serve --port 4000 should start the container with -p 4000:3000 and print http://localhost:4000. openhands serve with no flag keeps the previous default of 3000.

Added a unit test (test_launch_gui_server_custom_port) asserting the docker command maps 4000:3000, and updated the existing test_main_serve_calls_launch_gui_server parametrization to cover the new kwarg. Full test suite run locally: 1402 passed.

Type

• Feature

Notes

No change to default behavior; --port is optional and defaults to 3000.

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