Add ty type checker to CI and fix type errors (fixes bug from PR #157) (#192)

* Add ty type checker to CI and fix type errors

- Add ty (Astral's fast Python type checker) to GitHub CI workflow
- Fix type annotations across all RAG apps:
  - Update load_data return types from list[str] to list[dict[str, Any]]
  - Fix base_rag_example.py to properly handle dict format from create_text_chunks
- Fix type errors in leann-core:
  - chunking_utils.py: Add explicit type annotations
  - cli.py: Fix return type annotations for PDF extraction functions
  - interactive_utils.py: Fix readline import type handling
- Fix type errors in apps:
  - wechat_history.py: Fix return type annotations
  - document_rag.py, code_rag.py: Replace **kwargs with explicit arguments
- Add ty configuration to pyproject.toml

This resolves the bug introduced in PR #157 where create_text_chunks()
changed to return list[dict] but callers were not updated.

* Fix remaining ty type errors

- Fix slack_mcp_reader.py channel parameter can be None
- Fix embedding_compute.py ContextProp type issue
- Fix searcher_base.py method override signatures
- Fix chunking_utils.py chunk_text assignment
- Fix slack_rag.py and twitter_rag.py return types
- Fix email.py and image_rag.py method overrides

* Fix multimodal benchmark scripts type errors

- Fix undefined LeannRetriever -> LeannMultiVector
- Add proper type casts for HuggingFace Dataset iteration
- Cast task config values to correct types
- Add type annotations for dataset row dicts

* Enable ty check for multimodal scripts in CI

All type errors in multimodal scripts have been fixed, so we can now
include them in the CI type checking.

* Fix all test type errors and enable ty check on tests

- Fix test_basic.py: search() takes str not list
- Fix test_cli_prompt_template.py: add type: ignore for Mock assignments
- Fix test_prompt_template_persistence.py: match BaseSearcher.search signature
- Fix test_prompt_template_e2e.py: add type narrowing asserts after skip
- Fix test_readme_examples.py: use explicit kwargs instead of **model_args
- Fix metadata_filter.py: allow Optional[MetadataFilters]
- Update CI to run ty check on tests

* Format code with ruff

* Format searcher_base.py

tests/test_prompt_template_e2e.py

@@ -67,7 +67,7 @@ def check_lmstudio_available() -> bool:
         return False
 
 
-def get_lmstudio_first_model() -> str:
+def get_lmstudio_first_model() -> str | None:
     """Get the first available model from LM Studio."""
     try:
         response = requests.get("http://localhost:1234/v1/models", timeout=5.0)
@@ -91,6 +91,7 @@ class TestPromptTemplateOpenAI:
         model_name = get_lmstudio_first_model()
         if not model_name:
             pytest.skip("No models loaded in LM Studio")
+        assert model_name is not None  # Type narrowing for type checker
 
         texts = ["artificial intelligence", "machine learning"]
         prompt_template = "search_query: "
@@ -120,6 +121,7 @@ class TestPromptTemplateOpenAI:
         model_name = get_lmstudio_first_model()
         if not model_name:
             pytest.skip("No models loaded in LM Studio")
+        assert model_name is not None  # Type narrowing for type checker
 
         text = "machine learning"
         base_url = "http://localhost:1234/v1"
@@ -271,6 +273,7 @@ class TestLMStudioSDK:
         model_name = get_lmstudio_first_model()
         if not model_name:
             pytest.skip("No models loaded in LM Studio")
+        assert model_name is not None  # Type narrowing for type checker
 
         try:
             from leann.embedding_compute import _query_lmstudio_context_limit