Fix Duco diagnostics crash on connection error

[ronaldvdmeer]

Proposed change

The Duco diagnostics platform crashed with an unhandled DucoConnectionError when the device was unreachable during a diagnostics download. The exception propagated all the way up instead of returning a meaningful error to the user.

This PR wraps each API call in async_get_config_entry_diagnostics with a try/except that catches DucoConnectionError and raises HomeAssistantError, which Home Assistant handles gracefully.

Important

This fixes a crash in the diagnostics platform when the device is temporarily offline.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR is a beta fix for the Duco integration introduced in: Add target flow level and mode end time sensors to Duco integration #169298

Checklist

• I understand the code I am submitting and can explain how it works.
• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• I have followed the perfect PR recommendations
• The code has been formatted using Ruff (ruff format homeassistant tests)
• Tests have been added to verify that the new code works.
• Any generated code has been carefully reviewed for correctness and compliance with project standards.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies a diff between library versions and ideally a link to the changelog/release notes is added to the PR description.

[Copilot]

Pull request overview

Fixes Duco diagnostics downloads failing with an unhandled connection exception when the device is offline, by converting connection failures into a Home Assistant–native exception and adding a regression test.

Changes:

• Catch DucoConnectionError during diagnostics retrieval and raise HomeAssistantError instead.
• Add a translatable exception message for diagnostics connection failures.
• Add a test that verifies a diagnostics request returns an HTTP 500 when any of the underlying diagnostics API calls fails with a connection error.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
homeassistant/components/duco/diagnostics.py	Adds connection-error handling around diagnostics API calls.
homeassistant/components/duco/strings.json	Adds a new translated exception message for diagnostics connection failures.
tests/components/duco/test_diagnostics.py	Adds a regression test for connection errors during diagnostics download.

[erwindouna]

Good fix. Just a few points for discussion or patching. :)

[home-assistant]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

[erwindouna]

Looks good to me, thanks @ronaldvdmeer!