fix: error response for MCP

[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 a crash in call_mcp_tool_lob and call_mcp_tool_customer when an MCP tool returns an error response (isError=True).

Previously, when an MCP tool signalled a business error via the MCP protocol error flag, the SDK only logged the error and silently returned the error text as a successful result. This caused two problems:

1. AttributeError: 'NoneType' object has no attribute 'isError' — when langchain-mcp-adapters (≥0.3.x) intercepts a CallToolResult with isError=True and raises a ToolException before returning to the SDK, the SDK received None from session.call_tool() and crashed on the .content access.
2. Silent failure masking — even without langchain-mcp-adapters, an MCP tool error was silently swallowed and returned to the agent as a normal string, making the agent believe the tool call succeeded.

Fix: Both call_mcp_tool_lob (_lob.py) and call_mcp_tool_customer (_customer.py) now:

• Guard against a None result from session.call_tool() and raise AgentGatewayServerError with a descriptive message.
• Raise AgentGatewayServerError (instead of logging) when mcp_is_error(result) is True, so the error propagates correctly up to agw_client.call_mcp_tool() where it is caught and re-raised as AgentGatewaySDKError.

Related Issue

Closes #

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. Set up an agent that calls an MCP tool which returns an error response from the backend (e.g. an invalid change number in a BOM update).
2. Without this fix, the call raises AttributeError: 'NoneType' object has no attribute 'isError' (when using langchain-mcp-adapters) or silently returns the error text as a success.
3. With this fix, the call raises AgentGatewaySDKError: Tool invocation failed for '<name>': Tool '<name>' on '<url>' returned an error: <error text> — the agent framework receives a proper exception and can handle or surface it to the user.

Unit tests:

uv run pytest tests/agentgateway/unit/test_lob.py::TestCallMcpToolLob \
              tests/agentgateway/unit/test_customer.py::TestCallMcpToolCustomer -v

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

The AgentGatewayServerError exception class already existed and was already documented as the correct exception for isError=True tool results (exceptions.py:33). This fix aligns the actual behavior with the documented intent.

The outer except Exception handler in AgwClient.call_mcp_tool (agw_client.py) catches AgentGatewayServerError and wraps it in AgentGatewaySDKError with a Tool invocation failed for '<name>' prefix, so callers only need to handle AgentGatewaySDKError.