refactor is_active check and add update_status hook

[Thanhphan1147]

On update_status, check if the jenkins-agent service is running and if that's not the case, set the charm to error state to notify the operator.

Overview

Rationale

Juju Events Changes

Module Changes

Library Changes

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation is generated using src-docs
• The documentation for charmhub is updated
• The PR is tagged with appropriate label (urgent, trivial, complex)

The merge-base changed after approval.

[github-actions]

Test coverage for d5dfb73

Name                    Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------------
src/agent_observer.py      42      0      4      0   100%
src/charm.py               62      0     10      0   100%
src/charm_state.py         59      4      8      1    93%   114, 172-174
src/service.py             94      5     14      5    91%   94-96, 108->120, 135, 167, 190->194, 192->190
-------------------------------------------------------------------
TOTAL                     257      9     36      6    95%

Static code analysis report

Run started:2024-03-21 10:27:25.640167

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1113
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):

[yanksyoon]

LGTM, minor nitpick that can be implemented later, thanks!