[metricbeat/zookeeper] Fix potential panics and remove unused code in server module

[shmsr]

Proposed commit message

This PR improves the robustness of the Zookeeper server by adding proper bounds checking to prevent potential index-out-of-range panics, removes unused regex variables, and adds test coverage for edge cases.

• Bug Fixes:
  • Added bounds checking for versionCapturer.FindStringSubmatch() results before accessing the capture group, preventing panic on malformed version lines
  • Added bounds checking for dateCapturer.FindStringSubmatch() results before accessing the capture group, preventing panic on malformed date lines
  • Fixed off-by-one error in mode line parsing: changed len(modeSplit) < 1 to len(modeSplit) < 2 since the code accesses modeSplit[1]
• Code Cleanup:
  • Removed 4 unused regex variables: ipCapturer, thatNumberCapturer, portCapturer, dataCapturer

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

[github-actions]

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @shmsr? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[shmsr]

@mykola-elastic Could you please review this as well?