Improve Monzo API errors, icon, and test coverage

[JakeMartin-ICL]

Breaking change

Proposed change

This makes several small improvements to the Monzo integration:

• Raise translated update failures when the Monzo API returns an invalid response, while retaining detailed response data in debug logs.
• Give transaction event entities a transaction-specific icon.
• Add coverage for invalid API responses and account owner metadata edge cases.

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

Improves Monzo integration diagnostics, transaction presentation, and edge-case coverage.

Changes:

• Adds translated coordinator errors with debug-only API response details.
• Adds a transaction-specific event icon.
• Expands invalid-response and account-owner tests.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
homeassistant/components/monzo/coordinator.py	Raises translated update failures.
homeassistant/components/monzo/strings.json	Defines new error translations.
homeassistant/components/monzo/icons.json	Adds the transaction event icon.
tests/components/monzo/test_sensor.py	Tests invalid API responses and availability.
tests/components/monzo/test_helpers.py	Tests owner metadata edge cases.

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