fix: support additional Tavily search parameters via configuration to fix #548 (#643)

* fix: support additional Tavily search parameters via configuration to fix #548

- Add include_answer, search_depth, include_raw_content, include_images, include_image_descriptions to SEARCH_ENGINE config
- Update get_web_search_tool() to load these parameters from configuration with sensible defaults
- Parameters are now properly passed to TavilySearchWithImages during initialization
- This fixes 'got an unexpected keyword argument' errors when using web_search tool
- Update tests to verify new parameters are correctly set

* test: add comprehensive unit tests for web search configuration loading

- Add test for custom configuration values (include_answer, search_depth, etc.)
- Add test for empty configuration (all defaults)
- Add test for image_descriptions logic when include_images is false
- Add test for partial configuration
- Add test for missing config file
- Add test for multiple domains in include/exclude lists

All 7 new tests pass and provide comprehensive coverage of configuration loading
and parameter handling for Tavily search tool initialization.

* test: verify all Tavily configuration parameters are optional

Add 8 comprehensive tests to verify that all Tavily engine configuration
parameters are truly optional:

- test_tavily_with_no_search_engine_section: SEARCH_ENGINE section missing
- test_tavily_with_completely_empty_config: Entire config missing
- test_tavily_with_only_include_answer_param: Single param, rest default
- test_tavily_with_only_search_depth_param: Single param, rest default
- test_tavily_with_only_include_domains_param: Domain param, rest default
- test_tavily_with_explicit_false_boolean_values: False values work correctly
- test_tavily_with_empty_domain_lists: Empty lists handled correctly
- test_tavily_all_parameters_optional_mix: Multiple missing params work

These tests verify:
- Tool creation never fails regardless of missing configuration
- All parameters have sensible defaults
- Boolean parameters can be explicitly set to False
- Any combination of optional parameters works
- Domain lists can be empty or omitted

All 15 Tavily configuration tests pass successfully.

conf.yaml.example

@@ -55,3 +55,17 @@ BASIC_MODEL:
 #   # Exclude results from these domains
 #   exclude_domains:
 #     - example.com
+#   # Include an answer in the search results
+#   include_answer: false
+#   # Search depth: "basic" or "advanced"
+#   search_depth: "advanced"
+#   # Include raw content from pages
+#   include_raw_content: true
+#   # Include images in search results
+#   include_images: true
+#   # Include descriptions for images
+#   include_image_descriptions: true
+#   # Minimum score threshold for results (0-1)
+#   min_score_threshold: 0.0
+#   # Maximum content length per page
+#   max_content_length_per_page: 4000