[test] Add tests for server.registerSysTools

[github-actions]

Test Coverage Improvement: registerSysTools

Function Analyzed

• Package: internal/server
• Function: (*UnifiedServer).registerSysTools
• File: internal/server/unified.go:442
• Previous Coverage: 12.8%
• New Coverage: 80.9%
• Improvement: +68.1 percentage points
• Complexity: High — two handler closures (sysInitHandler and sysListHandler), each with multiple branches for argument parsing, session management, and backend delegation

Why This Function?

registerSysTools was selected as the most complex, under-tested function in the codebase. It contains two substantial closure-based handlers that together implement the sys___init and sys___list_servers MCP tools. These closures were almost entirely untested, even though they are the primary code path through which DIFC-enabled sessions are created and server lists are returned to clients.

Tests Added

New file: internal/server/register_sys_tools_test.go (11 test functions)

• ✅ TestRegisterSysTools_ToolsAreRegistered — both tools registered when EnableDIFC=true
• ✅ TestRegisterSysTools_NotRegisteredWhenDIFCDisabled — tools absent when DIFC is off (default)
• ✅ TestSysInitHandler_HappyPathWithToken — session created and token stored correctly
• ✅ TestSysInitHandler_HappyPathWithoutToken — success with empty/absent token
• ✅ TestSysInitHandler_EmptyTokenArg — explicit empty string token handled
• ✅ TestSysInitHandler_InvalidJSONArgs — malformed JSON returns error result
• ✅ TestSysInitHandler_DefaultSessionWhenNoContextSession — falls back to "default" key
• ✅ TestSysInitHandler_ReplacesExistingSession — re-init overwrites previous session token
• ✅ TestSysListHandler_HappyPath — success with pre-initialized session
• ✅ TestSysListHandler_SessionAutoCreated — success when requireSession auto-creates
• ✅ TestSysListHandler_DefaultContextSession — success with no explicit session in context
• ✅ TestRegisterSysTools_Idempotent — calling registerSysTools twice does not error

Coverage Report

Before: 12.8% (internal/server/unified.go:442: registerSysTools)
After:  80.9%
Improvement: +68.1pp
```

### Test Execution

All 11 new tests pass:

```
--- PASS: TestRegisterSysTools_ToolsAreRegistered (0.00s)
--- PASS: TestRegisterSysTools_NotRegisteredWhenDIFCDisabled (0.00s)
--- PASS: TestSysInitHandler_HappyPathWithToken (0.00s)
--- PASS: TestSysInitHandler_HappyPathWithoutToken (0.00s)
--- PASS: TestSysInitHandler_EmptyTokenArg (0.00s)
--- PASS: TestSysInitHandler_InvalidJSONArgs (0.00s)
--- PASS: TestSysInitHandler_DefaultSessionWhenNoContextSession (0.00s)
--- PASS: TestSysInitHandler_ReplacesExistingSession (0.00s)
--- PASS: TestSysListHandler_HappyPath (0.00s)
--- PASS: TestSysListHandler_SessionAutoCreated (0.00s)
--- PASS: TestSysListHandler_DefaultContextSession (0.00s)
--- PASS: TestRegisterSysTools_Idempotent (0.00s)
PASS
ok  	github.com/github/gh-aw-mcpg/internal/server

---

Generated by Test Coverage Improver
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