Register trigger platform upon use

[arturpragacz]

Proposed change

Register trigger platform upon use.

When a trigger is used, register its platform to make trigger descriptions available.

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 fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:
• Link to developer documentation pull request:
• Link to frontend pull request:

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.

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[Copilot]

Pull request overview

This PR ensures trigger platforms are registered when a trigger is first used, so the trigger-description system can discover and load the corresponding triggers.yaml metadata.

Changes:

• Register the trigger platform from within _async_get_trigger_platform() to populate hass.data[TRIGGERS] on-demand.
• Make _register_trigger_platform() idempotent to avoid re-registering existing triggers and to reduce misleading debug logs.
• Add a test covering trigger registration and subscriber notification behavior when a trigger platform is first accessed.

Reviewed changes

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

File	Description
homeassistant/helpers/trigger.py	Registers trigger platforms on-demand and makes platform registration idempotent.
tests/helpers/test_trigger.py	Adds coverage verifying registration side effects and subscriber notifications.