test: add comprehensive MCP test coverage for HIGH and MEDIUM risk gaps

Adds 34 new tests (47 total) across 6 test classes:
- TestToolSchemaGeneration: inputSchema required/optional, types, defaults
- TestToolInvocation: end-to-end calls for 8 different tools
- TestToolErrorHandling: nonexistent table/column, missing params, unknown tool
- TestResourceAccess: list/read resources, per-table URIs, name sanitization
- TestHelperEdgeCases: upstream/downstream, is_final, is_source, column counts
- TestCLI: transport, no-llm-tools flag, json vs directory path routing

Author: mingjerli