Fix stale pid handling in nebula.service

[officialasishkumar]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number: #5998

Description:

scripts/nebula.service start refuses to start when a stale pid file points to a live process, even if that pid has been reused by an unrelated executable. Users have to remove the stale pid file manually.

How do you solve it?

When a pid file exists, verify whether the live pid belongs to the expected Nebula executable. If it is the expected executable, keep the existing "already running" behavior. If it is positively identified as another executable, remove the stale pid file and continue startup. If the process cannot be identified, keep the conservative failure behavior.

Special notes for your reviewer, ex. impact of this fix, design document, etc:

Validated with bash -n scripts/nebula.service scripts/utils.sh, git diff --check, and a temporary service-layout repro where the pid file pointed to a live unrelated sleep process.

Checklist:

Tests:

• Unit test(positive and negative cases)
• Function test
• Performance test
• N/A

Affects:

• Documentation affected (Please add the label if documentation needs to be modified.)
• Incompatibility (If it breaks the compatibility, please describe it and add the label.）
• If it's needed to cherry-pick (If cherry-pick to some branches is required, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes:

Please confirm whether to be reflected in release notes and how to describe:

Fixed nebula.service start failing on stale pid files that point to unrelated live processes.

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