fix(debug): pin PORT env var so code-server binds to the correct port

[jeevb]

Problem

When the pod/image has a PORT environment variable set (common in Kubernetes), the code-server subprocess binds to the wrong port despite being launched with --bind-addr 0.0.0.0:{VSCODE_PORT}.

Root cause

code-server reads the PORT environment variable as one of its bind-address sources. The subprocess inherits the parent process's full environment, so an inherited PORT overrides the explicit --bind-addr flag.

Fix

• execute_command now accepts an optional env dict, merged on top of os.environ (preserving other inherited vars) and passed to asyncio.create_subprocess_shell(..., env=...).
• _start_vscode_server passes env={"PORT": str(VSCODE_PORT)} to the code-server subprocess, pinning PORT so an inherited value can't override --bind-addr. The hardcoded 6060 is also replaced with the existing VSCODE_PORT constant for consistency.

This ports flytekit PR flyteorg/flytekit#3450 to the async flyte-sdk debug helpers.

Additional fix (from code review)

The multiprocessing target was a lambda, which is not picklable under the spawn (macOS default) and forkserver start methods. Python 3.14 makes forkserver the default on Linux (POSIX), so child_process.start() would raise PicklingError and the code-server debug feature would fail in-container. The target is hoisted to a module-level _run_code_server function. (This also collapses a pre-existing double-asyncio.run bug in the lambda body.)

Tests

• test_execute_command_passes_env_to_subprocess — asserts the override reaches create_subprocess_shell while other inherited vars survive.
• test_execute_command_without_env_inherits_process_env — asserts env=None (inherit parent) when no override is given.
• test_start_vscode_server_pins_port_env_with_picklable_target — drives _start_vscode_server and asserts the child process pins PORT to VSCODE_PORT, the cmd uses --bind-addr 0.0.0.0:6060, and the target is picklable (guards against reintroducing a lambda).

All 23 tests in tests/flyte/test_debug.py pass; lint and format clean.