Keep managed Galaxy running after tests

[jmchilton]

Depends on #1701. Addresses #1175 and supersedes the implementation approach explored in #1185.

Motivation

planemo test normally tears down its managed Galaxy as soon as testing finishes. That remains the right default for automation, but it makes an interactive debugging loop awkward: the histories and datasets that explain a failure disappear with the server.

#1185 demonstrated the value of a test --serve workflow, but its implementation forked after Galaxy had started threads, switched the test process to the external-Galaxy engine, bypassed temporary-directory cleanup, and needed special handling for directory runnables. The engine and process-management layers have changed substantially since then.

This follow-up is built directly on #1701 so the package-installed Galaxy case uses the same Gravity-managed lifecycle as its serve, run, and ordinary test paths.

Changes

• Add planemo test --serve for Planemo-managed Galaxy engines.
• Keep one managed Galaxy context alive across test execution, report generation, and interactive inspection.
• Preserve embedded Galaxy tool-test histories while serving; the ordinary test path retains its existing cleanup behavior.
• Write test reports and print the summary before waiting, then show the Galaxy URL and stop cleanly on Ctrl-C.
• Expose --host and --port on planemo test; --serve uses Galaxy's standard port 9090 unless a port is supplied, while ordinary tests continue to request a free port.
• Reject --serve before execution for engines whose server lifecycle Planemo does not own, including cwltool, Toil, and external Galaxy.

For example:

planemo test --engine installed_galaxy --serve path/to/tool.xml

The capability lives at the engine boundary. Checkout-backed, Dockerized, and #1701's installed_galaxy engines share the implementation through LocalManagedGalaxyEngine; the installed engine therefore continues to use Gravity exclusively for process management. There is no fork(), engine substitution, or second Galaxy lifecycle implementation.

Stacking note

The branch is based directly on #1701's head commit. Because that head branch lives in a fork, GitHub requires this upstream PR to target galaxyproject/planemo:master; until #1701 merges, the displayed diff includes the prerequisite commit. Once #1701 merges, this PR reduces naturally to the single test --serve commit.

Compatibility

This is opt-in. Without --serve, engine selection, temporary-port selection, history cleanup, reporting, and shutdown behavior are unchanged.

Validation

• 41 focused command, engine, history-lifecycle, and Run package-installed Galaxy through Gravity #1701 unit tests passed.
• The broader non-Galaxy command/engine slice passed (30 passed, 22 skipped); its network-backed CWL case was also rerun successfully with network access.
• The released-package installed_galaxy acceptance suite passed (4 passed, 1 Tool Shed test intentionally deselected), including a new end-to-end case that verifies:
  • the report exists before Planemo waits;
  • the Gravity-managed Galaxy remains reachable;
  • the named embedded-tool test history remains visible;
  • Ctrl-C returns the test status and removes the process group and listening socket.
• Repository-wide Black, isort, Ruff, and flake8 passed.
• Mypy passed for all changed Python files.