Prioritize MCP error status and keep type fixture runtime safe

[aruintelligence]

Changes

• Classify MCP isError: true as tool_error even when a result also claims input_required.
• Add a conflicting-status regression case and clarify the result-status contract.
• Keep deliberate TypeScript negative examples type-checked without executing them under Node versions that discover .mts files as tests.

Verification

• node --test test/mcp-call-tool-gate.test.js (5 passed)
• npm test (764 passed)
• git diff --check