Fix critical bugs and extend test coverage

[Copilot]

Five correctness bugs found during code review, plus coverage gaps in previously untestable error paths. Coverage: 62.8% → 69.1% (app), 83.3% → 91.7% (metrics).

Bug Fixes

• unloadProfile — swapped errors.Is arguments (critical)

  // Before — never true; "file not found" early-return never fired
  if _, err := os.Stat(filePath); errors.Is(os.ErrNotExist, err) {
  // After
  if _, err := os.Stat(filePath); errors.Is(err, os.ErrNotExist) {

• printLoadedProfiles — wrong slice pre-allocation

  // Before — produces len(p) empty-string entries, then appends real names on top
  loadedProfileNames := make([]string, len(p))
  // After
  loadedProfileNames := make([]string, 0, len(p))

• compareLocalFiles, CopyFile, areProfilesReadable — os.Exit(1) in library functions
  All three called os.Exit(1) on recoverable errors (bad read, missing source file, readdir failure), crashing the DaemonSet instead of propagating the error. Replaced with proper return false/nil, fmt.Errorf(...) so callers can log and retry on the next poll cycle.

Coverage Extensions

Enabled by the os.Exit removals:

• NewConfigFromEnv, newDefaultLogger: 0% → 100%
• loadNewProfiles: 0% → 24% (empty/missing configmap, missing kernel path, orphan unload)
• compareLocalFiles, CopyFile, areProfilesReadable error paths
• metrics.ProfileUpdated: 0% → 100%
• Additional pollProfiles and /healthz handler coverage

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