Feature/ab#2111 add shutdown inhibit endpoint

[flonix8]

Description

This adds an http endpoint to inhibit HCU-triggered shutdowns (e.g. to complete a maintenance task).
Also adds HTTP API tests and refactors existing hcu related tests.

Motivation and Context

How has this been tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[Copilot]

Pull request overview

Adds a shutdown inhibition feature to the HCU subsystem, exposing new HTTP endpoints to inhibit an in-progress (or future) HCU-triggered shutdown and to query shutdown status. This is supported by introducing a dedicated shutdown timer model and reorganizing/refactoring HCU-related tests.

Changes:

• Add ShutdownModel for tracking shutdown timing, inhibition, and remaining time.
• Extend HCU HTTP API with /shutdown/inhibit/{seconds} and /shutdown/status, plus new API tests using httpx.
• Refactor/move existing HCU monitor tests into a dedicated tests/hcu/ package with shared fixtures.

Reviewed changes

Copilot reviewed 10 out of 12 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/test_hcu_monitor.py	Removes old top-level HCU monitor tests (moved/refactored under tests/hcu/).
tests/hcu/test_shutdown.py	Adds unit tests for the new shutdown timing/inhibition model.
tests/hcu/test_hcu_monitor.py	Adds refactored async integration tests for shutdown/resume behavior.
tests/hcu/test_hcu_api.py	Adds FastAPI route tests for shutdown inhibit and status endpoints.
tests/hcu/conftest.py	Introduces shared HCU test fixtures (serial socket, default skill, tmp file).
tests/hcu/init.py	Establishes tests.hcu as a package.
pyproject.toml	Adds httpx to dev dependencies for ASGI/API testing.
poetry.lock	Updates lockfile to include httpx and dependency group metadata changes.
msu_manager/hcu/skill.py	Adds shutdown inhibit and shutdown status endpoints to the HCU API routes.
msu_manager/hcu/shutdown.py	Introduces ShutdownModel and ShutdownError.
msu_manager/hcu/controller.py	Integrates ShutdownModel into shutdown scheduling/cancel flow and exposes status/inhibit helpers.
msu_manager/config.py	Adds shutdown_inhibit_max_s configuration setting.

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