test: type-check test/components/embedders and add it to mypy config

[omborda2002]

Related Issues

• Part of Run typing checks on our test files #10396

Proposed Changes:

Next piece-meal step for #10396, this one covers test/components/embedders/.

• Added test/components/embedders/ to the test:types mypy paths in pyproject.toml
• Fixed the 21 mypy errors in that folder, preferring real fixes over ignores:
  • built a proper Usage object in test_openai_text_embedder.py instead of passing a raw dict
  • annotated the empty_list_input variables as list[Document]
  • added assert embedder.embedding_backend is not None after warm_up() in the sentence transformers tests that set .model.max_seq_length, since the attribute is Optional before warm up
  • used scoped # type: ignore[arg-type] / # type: ignore[call-arg] only where a test passes a wrong type on purpose to check the raised TypeError

How did you test it?

• hatch run test:types passes with the new folder included (no issues in 394 source files)
• hatch run test:unit test/components/embedders/ passes (163 tests)
• hatch run fmt is clean

Notes for the reviewer

Tests and CI config only, so I did not add a release note file (same as #11728).

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have added unit tests and updated the docstrings.
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I have documented my code.
• I have added a release note file, following the contributors guidelines.
• I have run pre-commit hooks and fixed any issue.

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