[test] Add tests for logger.ServerFileLogger.Close

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Test Coverage Improvement: ServerFileLogger.Close

Function Analyzed

• Package: internal/logger
• Function: ServerFileLogger.Close
• File: internal/logger/server_file_logger.go:108
• Previous Coverage: 58.8%
• New Coverage: 94.1%
• Complexity: High (nil-receiver guard, loop over file map, sync+close error handling with first-error-wins tracking)

Why This Function?

ServerFileLogger.Close had the lowest real test coverage (58.8%) among non-env-gated functions in the testable subset of the codebase. Its complexity comes from:

• Nil-receiver guard (callable on nil *ServerFileLogger)
• Iterating a map of open log files, running two operations per file
• First-error-wins tracking: firstErr captures the first Sync or Close failure without being overwritten by later errors
• Unconditional map clearing even when errors occur

All of these paths except the "Sync succeeds but Close fails" branch (which requires NFS-level failure to trigger) are now exercised.

Tests Added

Test	Path Covered
TestServerFileLoggerClose_NilReceiver	Nil-receiver guard in Close()
TestServerFileLoggerClose_EmptyFiles	Loop body never entered; maps are still reset
TestServerFileLoggerClose_SyncError	file.Sync() error → firstErr set
TestServerFileLoggerClose_FirstErrorTracking	Multiple files both failing; first-error-wins over multiple iterations
TestServerFileLoggerClose_ValidFiles	Happy-path close (all files flush and close cleanly)
TestServerFileLoggerClose_CloseTwice	Idempotent: second call is a no-op with empty maps
TestServerFileLoggerLog_NilReceiver	Nil-receiver guard in Log()
TestServerFileLoggerLog_SyncError	file.Sync() error inside Log() does not panic
TestServerFileLoggerGetOrCreate_FileCreationError	os.OpenFile failure in getOrCreateLogger() gracefully falls back to LogDebug

Coverage Report

Before:
  Close             58.8%
  Log               84.6%
  getOrCreateLogger 90.9%

After:
  Close             94.1%   (+35.3 pp)
  Log              100.0%   (+15.4 pp)
  getOrCreateLogger 100.0%   (+9.1 pp)
```

Overall `internal/logger` package: **88.8% → 90.7%**

### Test Execution

```
=== RUN   TestServerFileLoggerClose_NilReceiver
--- PASS: TestServerFileLoggerClose_NilReceiver (0.00s)
=== RUN   TestServerFileLoggerClose_EmptyFiles
--- PASS: TestServerFileLoggerClose_EmptyFiles (0.00s)
=== RUN   TestServerFileLoggerClose_SyncError
--- PASS: TestServerFileLoggerClose_SyncError (0.00s)
=== RUN   TestServerFileLoggerClose_FirstErrorTracking
--- PASS: TestServerFileLoggerClose_FirstErrorTracking (0.00s)
=== RUN   TestServerFileLoggerClose_ValidFiles
--- PASS: TestServerFileLoggerClose_ValidFiles (0.00s)
=== RUN   TestServerFileLoggerClose_CloseTwice
--- PASS: TestServerFileLoggerClose_CloseTwice (0.00s)
=== RUN   TestServerFileLoggerLog_NilReceiver
--- PASS: TestServerFileLoggerLog_NilReceiver (0.00s)
=== RUN   TestServerFileLoggerLog_SyncError
--- PASS: TestServerFileLoggerLog_SyncError (0.00s)
=== RUN   TestServerFileLoggerGetOrCreate_FileCreationError
--- PASS: TestServerFileLoggerGetOrCreate_FileCreationError (0.00s)
PASS
ok      github.com/github/gh-aw-mcpg/internal/logger   0.006s

All existing tests continue to pass.

---

Generated by Test Coverage Improver
Next run should target writeToFile (63.6%) or logEntry (66.7%)

Generated by Test Coverage Improver · ◷