fix: filter **kwargs and *args from tool call schema generation

[Ricardo-M-L]

Summary

• Filter VAR_KEYWORD (**kwargs) and VAR_POSITIONAL (*args) parameter kinds from tool schema generation in autogen/tools/function_utils.py
• Fixed all 5 functions that iterate inspect.Signature.parameters: get_typed_signature(), get_param_annotations(), get_required_params(), get_default_values(), get_missing_annotations()
• Added 12 comprehensive tests covering: kwargs-only functions, args+kwargs, regular params with kwargs, and regression tests for functions without variadic params

Why

When a function has **kwargs, AG2's tool schema generation treats kwargs as a regular required parameter. The LLM sees it in the schema and nests all arguments inside a kwargs field, causing Pydantic validation failure. See the issue for a detailed reproduction.

Fixes #1790

Test plan

• All 12 new tests pass (test_get_typed_signature_filters_var_keyword, test_get_typed_signature_filters_var_positional, test_get_typed_signature_filters_both_var_positional_and_keyword, test_get_param_annotations_excludes_kwargs, test_get_required_params_excludes_kwargs, test_get_required_params_excludes_args, test_get_default_values_excludes_kwargs, test_get_missing_annotations_excludes_var_params, test_get_function_schema_with_kwargs_only, test_get_function_schema_with_regular_params_and_kwargs, test_get_function_schema_with_args_and_kwargs, test_get_function_schema_no_kwargs_regression)
• All 27 pre-existing tests still pass (1 async test fails due to missing pytest-asyncio — pre-existing, unrelated)
• Verified the exact reproduction case from the issue: hello_world_v3(arg1, arg2, **kwargs) now generates a schema without kwargs in properties or required

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Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
autogen/tools/function_utils.py	90.47% <100.00%> (ø)

... and 64 files with indirect coverage changes

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