test: type-check agent state tests

[KXHXK]

Why:

Continue #10396 with a small, reviewable increment by adding the Agent state tests to the repository's mypy target.

The file had three related errors because mypy inferred the generic type parameter of merge_lists() as Never when both arguments were lists. An explicit result type supplies the intended int context without changing runtime behavior.

What:

• add test/components/agents/test_state_class.py to the test:types mypy allowlist
• annotate the merged list result as list[int]

How can it be used:

This runs automatically as part of the existing hatch run test:types command.

How did you test it:

• uv run --with mypy mypy --install-types --non-interactive --cache-dir=.mypy_cache test/components/agents/test_state_class.py: no issues
• uv run --with pytest pytest test/components/agents/test_state_class.py -q: 52 passed
• uvx ruff check test/components/agents/test_state_class.py pyproject.toml: passed
• uvx ruff format --check test/components/agents/test_state_class.py: passed

Before the annotation, the focused mypy command reported three errors at the merge_lists call; after the annotation it reports none.

Checklist:

• I have read the contribution guidelines
• I have added or updated tests where needed
• I have run the relevant tests and quality checks
• This is a test/type-checking-only change; no release note is needed
• The commit includes DCO sign-off

AI assistance

Codex assisted with candidate screening, type-check diagnosis, and validation. I reviewed and understand the submitted change and all reported test results.

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