[ISD-3570] Flush runner will kill reactive processes as well

[yhaliaw]

Applicable spec:

Overview

• Flush runner will kill the reactive process.
• Retried reactive job messages are delayed by 60 secs before processing.
• Flush are done on upgrade charm.

Rationale

• Fix the issue where the old reactive process persist after a charm upgrade.
• Fix rapid retry of reactive job messages on error.

Checklist

• The charm style guide was applied.
• The contributing guide was applied.
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex).
• The changelog is updated with changes that affects the users of the charm.

[yanksyoon]

LGTM, some comments regarding the architecture which may require team coordination.

[github-actions]

Test results for commit 12a1b80

Test coverage for 12a1b80

Wrote XML report to coverage/coverage.xml

Static code analysis report

Run started:2025-07-21 00:47:24.641770

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 2024
  Total lines skipped (#nosec): 2
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 1

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
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