fix: mcp tool converter sending none values

[NicoleMGomes]

Disclaimer: Do not include SAP-internal or customer-specific information in this PR (e.g. internal system URLs, customer names, tenant IDs, or confidential configurations). This is a public repository.

Description

Fixes mcp_tool_to_langchain in the Agent Gateway converter so that optional MCP tool parameters are no longer forwarded to call_mcp_tool as None values, and adds an omit_none configuration option to control this behaviour.

Root cause: Optional fields in the generated Pydantic args_schema were typed as str | None with a plain None default. LangChain passes all schema fields — including optional ones the LLM did not supply — to the tool's run closure as keyword arguments. This caused call_mcp_tool to receive explicit None for every optional parameter, which breaks downstream MCP tool invocations that distinguish between an absent parameter and a null one.

Changes:

• Optional fields are now typed as str | None with Field(default=None) so Pydantic accepts None from LangChain's validation layer, while the field is still not in required.
• The run closure strips None values from kwargs before forwarding to call_tool (controlled by the new omit_none flag).
• New keyword-only parameter omit_none: bool = True on mcp_tool_to_langchain — set to False to forward None values explicitly to the MCP server.
• Test suite refactored: shared _make_tool() fixture removes duplication; new TestMcpToolToLangchainInvocation class covers end-to-end LangChain tool invocation including the omit_none=False path.
• Removed stale # ty: ignore comments from tests/adms/unit/test_client.py and tests/destination/unit/test_client.py that caused ty type-check pre-commit failures.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

How to Test

1. Install the package: uv sync
2. Run the converter unit tests:

   uv run pytest tests/agentgateway/unit/test_converters.py -v
   

3. Verify the default behaviour (None omitted):

   import asyncio
   from unittest.mock import AsyncMock
   from sap_cloud_sdk.agentgateway import MCPTool
   from sap_cloud_sdk.agentgateway.converters import mcp_tool_to_langchain
   
   tool = MCPTool(
       name="get_supplier_bid", server_name="ariba",
       description="Gets supplier bids",
       input_schema={
           "type": "object", "required": ["eventid"],
           "properties": {"eventid": {"type": "string"}, "showdeclinedreason": {"type": "string"}},
       },
       url="https://example.com/mcp",
   )
   call_tool = AsyncMock(return_value="ok")
   lc_tool = mcp_tool_to_langchain(tool, call_tool, lambda: "token")
   asyncio.run(lc_tool.arun({"eventid": "E001"}))
   assert "showdeclinedreason" not in call_tool.call_args.kwargs  # omitted, not None

4. Verify omit_none=False forwards None explicitly:

   call_tool2 = AsyncMock(return_value="ok")
   lc_tool2 = mcp_tool_to_langchain(tool, call_tool2, lambda: "token", omit_none=False)
   asyncio.run(lc_tool2.arun({"eventid": "E001", "showdeclinedreason": None}))
   assert call_tool2.call_args.kwargs["showdeclinedreason"] is None  # forwarded as None

5. Run the full pre-commit suite: uvx pre-commit run --all-files

Checklist

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

Additional Notes

omit_none is keyword-only (declared after *) to prevent positional misuse. The default True preserves existing behaviour — no changes needed for current callers.

The merge-base changed after approval.