Refactor routes and harden AddTool

[mk0walsk]

This PR refactors server.go to eliminate code duplication in route registration and hardens AddTool in types.go against silent failures.

Key Changes:

1. DRY & Consistency: Extracted identical route registration logic from setupSSETransport and setupStreamableTransport into a registerRoutes helper. This reduces duplication and ensures consistent timeout handling across transport modes.
2. Error Visibility: Added explicit error logging to AddTool when the server type assertion fails. Previously, passing a custom McpServer implementation would cause the tool registration to silently fail, leading to difficult debugging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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