fix(agentgateway): raise AgentGatewayServerError on isError tool results

[wangzhengzhuo05]

What

AgentGatewayClient.call_mcp_tool silently swallowed MCP tool errors. When the MCP server returns a spec-compliant error response (isError: true, HTTP 200, SSE event: message with result.isError == true), both call_mcp_tool_lob (LoB flow) and call_mcp_tool_customer (customer flow) logged the error and returned the raw error text as if it were a successful tool result. Callers could not distinguish success from failure — any caller inspecting result.isError on the return value crashed with AttributeError (reported as 'NoneType' object has no attribute 'isError').

Why

The SDK already documents the intended contract in src/sap_cloud_sdk/agentgateway/exceptions.py: AgentGatewayServerError is raised when "A tool invocation returns an error result (isError=True)". The implementation violated its own documented contract — the exception was never raised on this path. Silently returning a value that crashes .isError accessors is never a valid outcome for a tool call that failed server-side.

How

• src/sap_cloud_sdk/agentgateway/_lob.py: in call_mcp_tool_lob, replace the log-and-return block with raise AgentGatewayServerError(f"Tool '{tool.name}' on '{tool.url}' returned an error: {text}") when mcp_is_error(result) is true.
• src/sap_cloud_sdk/agentgateway/_customer.py: identical fix in call_mcp_tool_customer so both flows behave consistently.
• Success path unchanged (still returns the tool text); empty-content path unchanged (still returns "").
• Tests: added test_raises_server_error_on_is_error to TestCallMcpToolLob and TestCallMcpToolCustomer; set explicit is_error=False/isError=False on the existing success-path mocks (a plain MagicMock is truthy, so those tests were implicitly passing through the error branch before).

Verification

• Reproduced the bug at the wire level: a local HTTP server answering the exact SSE payload from the issue (event: message / data: {"result": {"content": [...], "isError": true}, "jsonrpc": "2.0", "id": 3}) against call_mcp_tool_lob returned the error text as a plain str and .isError raised AttributeError: 'str' object has no attribute 'isError'.
• Targeted suite: pytest tests/agentgateway/unit/test_lob.py tests/agentgateway/unit/test_customer.py → 103 passed.
• Full agentgateway unit suite: 220 passed, 24 failed — the 24 failures are in test_converters.py and pre-exist on a clean origin/main checkout (verified via a fresh worktree: same 24 failures, unrelated to this change).
• Mutation check: reverted only the fix's behavior in _lob.py (restored log-and-return) → test_raises_server_error_on_is_error fails (1 failed); restored the fix → passes. The new test guards this behavior.

Notes

• The reporter's exact stack ('NoneType' object has no attribute 'isError') comes from an older SDK version; on current main the returned value is a str, producing 'str' object has no attribute 'isError' — same class of defect, same fix.
• No new dependencies; no formatting churn.

Fixes #326

AI-generated code disclosure

This change was implemented with AI assistance (OpenCode + muse-spark) and reviewed/verified for correctness, security, and license compatibility per SAP GenAI guidelines.