Add tests for server tool_registry functions

Adds comprehensive tests for the previously untested registerAllTools,
registerAllToolsSequential, registerAllToolsParallel, registerSysTools,
registerSysTool, and callSysServer functions in internal/server/tool_registry.go.

Tests cover:
- DIFC enabled/disabled paths (sys tools registered only when DIFC is on)
- Sequential vs parallel backend launch modes
- Partial failure handling (continues when one backend fails)
- All-failure resilience (returns nil, logs warnings)
- Empty server list (no-op, no tools)
- sys tool metadata storage via registerSysTool
- callSysServer delegation to SysServer (sys_init, sys_list_servers, unknown)

A newMockBackend helper provides a reusable httptest.Server that responds
to initialize and tools/list requests, following the existing test patterns
in register_tools_from_backend_test.go.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]