[Backport 2025.4] fix(nemesis): prevent UnboundLocalError when exception raised before DisruptionEvent context

[scylladbbot]

Description

nemesis_event was accessed in finally block but only defined within with DisruptionEvent() context manager. When KillNemesis raised during check_cluster_health() before entering the with block, variable was undefined → UnboundLocalError.

Before:

def wrapper(*args, **kwargs):
    runner = caller_obj.runner
    try:
        # ... 
        runner.cluster.check_cluster_health()  # KillNemesis raised here
        
        with DisruptionEvent(...) as nemesis_event:  # Never reached
            # ...
    finally:
        runner.last_nemesis_event = nemesis_event  # UnboundLocalError

After:

def wrapper(*args, **kwargs):
    runner = caller_obj.runner
    nemesis_event = None  # Initialize early
    try:
        # ...
        runner.cluster.check_cluster_health()  # KillNemesis raised here
        
        with DisruptionEvent(...) as nemesis_event:  # Never reached
            # ...
    finally:
        if nemesis_event is not None:  # Safe access
            runner.last_nemesis_event = nemesis_event

Changes:

• sdcm/nemesis.py:6273: Initialize nemesis_event = None
• sdcm/nemesis.py:6431-6432: Guard assignment with None check

Testing

• Manual validation confirms fix prevents the UnboundLocalError
• Python syntax validation passes
• Note: Unit test for the wrapper function is not included as the function has complex dependencies and structure that make it difficult to test without significant refactoring

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)

Fixes: SCT-92

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• (cherry picked from commit a3c67c0)

Parent PR: #13628

[scylladbbot]

@copilot - This PR has conflicts, therefore it was moved to draft
Please resolve them and mark this PR as ready for review