Apply MCP code quality principles

[Gabriel-Darbord]

Summary

• trust repository/debugger collaborators more directly instead of catching broad errors around ordinary sends
• handle optional Iceberg metadata explicitly, including nil locations, detached heads, and no-commit repositories
• replace exception-driven debugger reference parsing with structural validation

Verification

• Pharo 13 test_run MCPTestCase: 632 selected, 631 ran, 631 passed, 1 expected skip
• Pharo 13 test_run MCPDebugToolContractTest: 59 passed
• git diff --check