[quality] tests: add selfUpdateFallback coverage via injectable execReplaceFunc

[kubestellar-hive]

Summary

Fixes the last 0%-coverage function in pkg/agent/updater not already addressed by open PRs (#20486 covers gitStash/gitStashPop, #20545 covers downloadFile).

Changes

• pkg/agent/updater/build.go: Extracts ExecReplace into a package-level execReplaceFunc variable so tests can override it without triggering syscall.Exec (which would replace the running test process on Unix).
• pkg/agent/updater/self_update_fallback_test.go (new): 3 unit tests for selfUpdateFallback:
  • Verifies killBackend and restartBackend are called
  • Verifies exec proceeds even after restartBackend error
  • Verifies the current binary path is passed to execReplaceFunc

Test plan

• go test ./pkg/agent/updater/... -run TestSelfUpdateFallback passes in CI

Fixes #20478

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🚫 Scanner Merge Guardrails: Rate Limit Exceeded

This PR cannot be auto-merged because the scanner has already merged 5 PRs in the last hour.

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Reason: This guardrail prevents rapid consecutive merges that can cause build instability.

See issue #18218 for context.

[github-actions]

🚫 Scanner Merge Guardrails: Rate Limit Exceeded

This PR cannot be auto-merged because the scanner has already merged 4 PRs in the last hour.

Rate Limit: 3 merges per hour
Cooldown: Wait until the rate limit window resets

Reason: This guardrail prevents rapid consecutive merges that can cause build instability.

See issue #18218 for context.