fix: eliminate duplicate sys tool delegation block via callSysServer helper

[Copilot]

Summary

Closes the duplicate code issue for the sys tool sysServer delegation block in internal/server/unified.go.

The two sys-tool handler closures (sysInitHandler and sysListHandler) previously contained an identical ~10-line block that:

1. Marshaled a tools/call params object with the tool name
2. Invoked us.sysServer.HandleRequest("tools/call", ...)
3. Checked the error
4. Returned nil, result, nil

Changes

• Extracted a callSysServer(toolName string) (interface{}, error) helper method on UnifiedServer that centralises the marshal → HandleRequest → error check → return pattern
• Updated sysInitHandler and sysListHandler to delegate to the helper, keeping only their tool-specific pre-call logic and log messages
• The helper makes it straightforward to add future sys_* tools without copy-pasting the delegation block

Implementation Checklist

• Review duplication findings
• Extract callSysServer(toolName string) helper on UnifiedServer
• Update sysInitHandler to use helper
• Update sysListHandler to use helper
• Verify future sys_* tools follow the helper pattern
• Tests pass (make agent-finished)

Resolves: [duplicate-code] Duplicate Code Pattern: Sys Tool sysServer Delegation Block

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