Clean up template engine after extension modularization

[frenck]

Proposed change

Small cleanup after the template engine modularization effort:

• Remove redundant import collections.abc from the state extension (already importing Iterable from collections.abc directly)
• Move remaining state infrastructure tests (test_invalid_entity_id, test_length_of_states, test_slice_states, test_state_attributes, test_unavailable_states) from test_init.py to test_states.py where they belong

There is no functional change in this PR.

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:
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• 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.
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• There is no commented out code in this PR.
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• 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.

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[Copilot]

Pull request overview

This PR performs small cleanup work after template engine modularization by removing a redundant import in the state template extension and relocating state-related template tests into the more appropriate test module.

Changes:

• Remove redundant collections.abc import and use the already-imported Iterable in StateExtension.expand.
• Move state infrastructure template tests from test_init.py into test_states.py (no behavioral changes intended).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/helpers/template/test_states.py	Adds the moved state-related template tests and required imports/helpers.
tests/helpers/template/test_init.py	Removes state-related tests that were relocated to test_states.py.
homeassistant/helpers/template/extensions/state.py	Simplifies imports and updates isinstance(..., Iterable) usage accordingly.